Abstract:
Embodiments of the present invention provide a system and method of providing analog setpoints that eliminate, or at least substantially reduces, the shortcomings of prior art analog setpoint systems and methods. One embodiment of the present invention includes a method of multiplexing analog setpoints comprising transmitting the analog signal to a plurality of target devices, wherein the analog signal represents multiple setpoints, transmitting a first setpoint indicator separate from the analog signal to indicate to a first target device that a first setpoint for the first target device is being represented by the analog signal, saving a first setpoint value asserted by the analog signal at the first target device in response to the first setpoint indicator.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     Embodiments of the present invention relate to systems and methods for asserting setpoints. More particularly, embodiments of the present invention relate to systems and method for asserting analog setpoints. Even more particularly, embodiments of the present invention relate to systems and methods for multiplexing multiple analog setpoints on an analog communications link.  
       BACKGROUND  
       [0002]     Many control devices rely on analog setpoints to indicate a desired state to which a system should be controlled. An analog setpoint is typically a voltage or current applied to a controller that represents a desired value of a measured parameter. The voltage/current may represent a desired value of a temperature, motor speed, pressure, pressure differential, temperature differential or other parameter. The analog setpoint is typically digitized at the controller and converted to a setpoint value for the parameter. The setpoint value can be compared to measured values of the parameter for control purposes. For example, a temperature controller can receive an analog signal of 2.2 Volts, digitize the signal and convert the value to 20 degrees Celsius. The controller can then compare the measured temperature values in a system to determine if the temperature needs to be raised or lowered to reach 20 degrees Celsius. A variety of control schemes, including proportional control schemes, proportional integral, proportional integral derivative, fuzzy logic control schemes are known for controlling a process parameter based on a setpoint.  
         [0003]     Many existing controllers only have one or a limited number of analog ports available over which to send or receive a setpoint signal. For a controller that asserts analog setpoints to other controllers, this limits the number of devices it can control. In other words, the number of slave controllers to which a particular master controller can assert setpoints is limited to the number of analog ports at the master controller. Additionally, for each controller to which a setpoint is asserted, a separate analog communications link is required.  
       SUMMARY OF THE INVENTION  
       [0004]     Embodiments of the present invention provide a system and method of providing analog setpoints that eliminate, or at least substantially reduces, the shortcomings of prior art analog setpoint systems and methods. One embodiment of the present invention includes a method of multiplexing analog setpoints comprising transmitting the analog signal to a plurality of target devices, wherein the analog signal represents multiple setpoints, transmitting a first setpoint indicator separate from the analog signal to indicate to a first target device that a first setpoint for the first target device is being represented by the analog signal, saving a first setpoint value asserted by the analog signal at the first target device in response to the first setpoint indicator.  
         [0005]     Another embodiment of the present invention includes a system for multiplexing analog setpoints comprising a master controller, a plurality of slave controllers connected to the master controller, an analog communications link connecting the plurality of slave controllers to the master controller and one or more digital communications links connecting the plurality of slave controllers to the master controller. The master controller is operable to transmit an analog signal on the analog communications link representing a plurality of analog setpoints, wherein the plurality of setpoints are time multiplexed in the analog signal, transmit a first setpoint indicator on at least one of the digital communications links to the first slave controller in a first period of time and transmit a second setpoint indicator on at least one of the digital communications links to a second slave controller in a second period of time. The analog signal, according to one embodiment, represents a first setpoint in the first period of time and a second setpoint in the second period of time.  
         [0006]     Yet another embodiment of the present invention includes a computer program product comprising a set of computer instructions stored on a computer readable medium. The set of computer instructions further comprising instructions executable by a processor to transmit a setpoint signal over a first communications link, wherein the setpoint signal multiplexes a plurality of setpoints, transmit a first setpoint indicator signal to a first target device to indicate to the first target device that the setpoint signal represents a setpoint for the first target device in a first period of time and transmit a second setpoint indicator signal to a second target device to indicate to the second target device that the setpoint signal represents a setpoint for the second target device in a second period of time.  
         [0007]     The present invention provides an advantage over prior art systems and methods of asserting analog setpoints by allowing multiple analog setpoints to be asserted on a common analog communications link.  
         [0008]     Embodiments of the present invention provide another advantage over prior art systems by allowing a controller to connect to assert analog setpoints to multiple other controllers using a single or a limited number of analog ports.  
         [0009]     In addition, embodiments of the present invention provide another advantage by reducing the amount of analog cabling required in systems with multiple controllers.  
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0010]     A more complete understanding of the present invention and the advantages thereof may be acquired by referring to the following description, taken in conjunction with the accompanying drawings in which like reference numbers indicate like features and wherein:  
         [0011]      FIG. 1  is a diagrammatic representation of one embodiment of a system for mixing fluids;  
         [0012]      FIGS. 2A and 2B  provide flow charts illustrating one embodiment of a method for controlling flow of fluids to create a mixed fluid;  
         [0013]      FIG. 3  is a diagrammatic representation of another embodiment of a system for mixing fluids;  
         [0014]      FIGS. 4A-4C  provide flow charts illustrating one embodiment of another method for controlling flow of fluids to create a mixed chemical;  
         [0015]      FIG. 5  is a diagrammatic representation of yet another embodiment of system for mixing fluids;  
         [0016]      FIGS. 6A-6C  provide flow charts illustrating another embodiment of another method for controlling flow of fluids to create a mixed chemical;  
         [0017]      FIGS. 7A-7F  provide diagrammatic representations of one embodiment of a static mixer assembly  700  and its components;  
         [0018]      FIGS. 8A-8C  provide diagrammatic representations of another embodiment of a mixer assembly;  
         [0019]      FIG. 9  is a diagrammatic representation of one embodiment of a system for multiplexing analog set points;  
         [0020]      FIG. 10  is a diagrammatic representation of an analog setpoint signal and corresponding setpoint indicator signals;  
         [0021]      FIG. 11  is a diagrammatic representation of one embodiment of a system for multiplexing analog setpoints;  
         [0022]      FIG. 12  is a diagrammatic representation of an analog setpoint signal and corresponding signals for asserting setpoint indicators; and  
         [0023]      FIG. 13  is a flow chart illustrating one embodiment of multiplexing analog setpoints.  
     
    
     DETAILED DESCRIPTION  
       [0024]     Preferred embodiments of the invention are illustrated in the FIGURES, like numerals being used to refer to like and corresponding parts of the various drawings.  
         [0025]     Embodiments of the present invention provide a system and method for multiplexing analog setpoints. According one embodiment of the present invention, an analog signal source (e.g., a master controller) can assert an analog signal to multiple target devices (e.g., slave controllers) on a common analog communications link. The analog signal can represent a plurality of setpoints. According to one embodiment, setpoint indicators can be asserted to the target devices on digital communications links. When a particular target device receives a setpoint indicator, the target device can save the value of the analog setpoint signal for use as a setpoint. It should be noted that while embodiments of the present invention will be discussed in terms of controllers used in a fluid mixing system, embodiments of the present invention are applicable to any system requiring assertion of multiple analog setpoints.  
         [0026]      FIG. 1  is a diagrammatic representation of one embodiment of a system  100  for mixing fluids. System  100  includes two flow controllers  102  and  104  that are in fluid communication with a mixer  106 . System  100  further includes a temperature sensor  108  upstream of flow controller  102 , a temperature sensor  110  upstream of flow controller  104  and a temperature sensor  112  downstream of mixer  106 . Temperature sensor  108  and temperature sensor  110  are connected to (i.e., can communicate a signal representative of temperature) at least one of the flow controllers; flow controller  104  in this example. Temperature sensor  112  is also connected to at least one of the flow controllers. In this example, temperature sensor  112  is connected to flow controller  102 .  
         [0027]     According to one embodiment, flow controller  102  and flow controller  104  is each an OptiChem P1200 LFC flow controller produced by Mykrolis Corporation of Billerica, Mass. (now part of Entegris, Inc. of Chaska, Minn.), though other suitable flow controllers can be utilized. Mixer  106  can include any suitable dynamic or static mixer for mixing fluid flows. One embodiment of a static mixer is described in conjunction with  FIGS. 7A-7F . The temperature sensors  108 ,  110  and  112  can include any suitable temperature sensors.  
         [0028]     Fluid that is hotter than a target temperature (e.g., hot fluid  114 ) is supplied to flow controller  102  and a fluid that is colder than a target temperature (e.g., cold fluid  116 ) is supplied to flow controller  104 . Flow controller  102  regulates the flow of hot fluid  114  and flow controller  104  regulates the flow of cold fluid  116  to mixer  106 . These fluids are blended at mixer  106  to produce mixed fluid  118  at a desired temperature and flow rate.  
         [0029]     The flow rates of hot fluid  114  and cold fluid  116  to mixer  106  can be controlled based on a target temperature (e.g., of mixed fluid  118 ), the temperatures of the hot and cold fluids, the fluid properties of the hot and cold fluids and the measured temperature of mixed fluid  118 . More particularly, a process tool, control computer or other system can provide flow controller  104  a target temperature (t T1 ) and flow rate (Q T1 ) of mixed fluid  118 . Additionally, temperature sensor  108  provides the temperature of hot fluid  114  (t H ) and temperature sensor  110  provides the temperature of cold fluid  116  (t C ). Flow controller  102  and flow controller  104  can also be provided with or preprogrammed with the type of hot and/or cold fluid used in system  100 .  
         [0030]     Based on the fluid type and temperatures of hot fluid  114  and cold fluid  116 , flow controller  102  can calculate the densities (ρ H , ρ C )and specific heats (Cp H , Cp C ) of hot fluid  114  and cold fluid  116 . Flow controller  104  can similarly determine the density (PT) and specific heat (CP T ) of mixed fluid  118  at the target temperature (t T ). For example, if each of hot fluid  114  and cold fluid  116  is D.I. H 2 O, the densities and specific heats can be calculated based on polynomials using the following coefficients:  
                       TABLE 1                       Order   ρ = f (t)   Cp = f (t)                   0   .99988   1.00919       1   6.20242E−05   −9.50319E−04       2   −8.37727E−06     2.8655E−05       3   6.62195E−08   −4.28993E−07       4   −4.17404E−10     3.44932E−09       5   1.15955E−12   −1.10643E−11                  
 
         [0031]     Table 1 is provided by way of example and not limitation. Other equations, lookup tables or other suitable mechanism can be used to determine the specific heat and density for hot fluid  114 , cold fluid  116  and mixed fluid  118 . Moreover, it should be understood that hot fluid  114  and cold fluid  116  can be different fluids.  
         [0032]     Using the target flow rate (Q T1 ), target temperature (t T1 ), hot fluid temperature (t H ), cold fluid temperature (t C ), specific heats of the hot, cold and mixed fluids (Cp H , Cp C , Cp T )and densities of the hot and cold fluids (p H , P C ), controller  104 , according to one embodiment, can calculate the target flow rate of cold fluid  116  (Q C ) to mixer  106  based, for example, on the following equation:
 
 Q   C   =Q   T *(1000/60)*(ρ C /ρ T )*( t   H   *Cp   H   −t   T   Cp   T )/( t   H   *Cp   H   −t   C   *Cp   C )  [EQN. 1] 
 
                                                       Q T  = target flow rate   (lpm)           t T  = target temperature   (° C.)           t H  = hot fluid temperature   (° C.)           t C  = cold fluid temperature   (° C.)           ρ C  = cold fluid density   (g/cm 3 )           ρ H  = hot fluid density   (g/cm 3 )           Cp C  = cold fluid specific heat   (cal/g*° C.)           Cp H  = hot fluid specific heat   (cal/g*° C.)           Cp T  = mixed fluid specific heat at t T     (cal/g*° C.)                      
 
 Continuing with the previous example, Q T =Q T1  and t T =t T1 , and flow controller  104  can determine the appropriate Q C  according to any mechanism known or developed in the art. Flow controller  104  can regulate the flow of cold fluid  116  to the rate of Q C  (within the tolerances of flow controller  104 ) using pressure differential based flow control, heat loss based flow control or other flow control scheme. 
 
         [0033]     Flow controller  104  can further pass a temperature set point t SP  to controller  102 . The temperature set point, in this case, can indicate the desired temperature of mixed fluid  118 . For example, t SP  can be equal to t T . Controller  102  compares the temperature of the mixed fluid (t M1 ) to t SP . If t M1 &gt;than t SP , controller  104  can decrease the flow of hot fluid  114  and if t M1 &lt;t SP , controller  104  can increase the flow of hot fluid  114 . By adjusting the flow of hot fluid, t M1  will approach t SP . When t M1  is approximately equal to t SP , (i.e., within an acceptable deviation (e.g. 5%)), this indicates that mixed fluid  118  has reached the target flow rate and target temperature. In another embodiment, flow controller  104  receives t M1  from temperature sensor  112  and passes t M1  and t SP  to flow controller  102 .  
         [0034]     Controller  104  can continually recalculate Q C  and t SP  (e.g., approximately at 1 Hz or above, according to one embodiment) as the input fluid temperatures, desired mixed fluid flow rate or other parameters change. Thus, the present invention can quickly adjust to changing process parameters.  
         [0035]     As described above, controller  104  and controller  102  act in a master-slave fashion with controller  104  providing t SP  to controller  102 . The master-slave dynamic of these controllers can be reversed with controller  102  processing the inputs providing a t SP  to controller  104 . Furthermore, one of the controllers can be provided with the target temperature and flow rate and the other controller can be provided with t SP  from an outside computer system or tool. In this case, neither controller  102  nor controller  104  acts as a master or slave with respect to the other controller.  
         [0036]     It should be noted that higher temperature fluids can cause errors in pressure based controllers. If a pressure based flow controller is used to control the hot DIW, significant errors may be encountered as commonly used pressure sensors are typically sensitive to temperature changes. If the hot fluid flow controller controls flow based on pressure, temperature compensation circuitry can be used. Or, as in the embodiments described above, the hot fluid flow controller can employ a temperature based control scheme.  
         [0037]      FIGS. 2A and 2B  provide flow charts illustrating one embodiment of a method for controlling flow of fluids to create a mixed fluid. The method of  FIGS. 2A and 2B  can be implemented as computer instructions that are executable by a processor stored on a computer readable medium. For example, embodiments of the present invention can be implemented through programming of one or more OptiChem P1200 LFC flow controllers.  
         [0038]     The flow chart of  FIG. 2A  corresponds to the control method implemented at the cold fluid flow controller (e.g., flow controller  104  of  FIG. 1 ) and  FIG. 2B  corresponds to the method implemented at the hot fluid flow controller (e.g., flow controller  102  of  FIG. 1 ).  
         [0039]     The cold fluid flow controller receives inputs including the target mixed fluid temperature (t T1 ), the target mixed fluid flow rate (Q T1 ), the cold fluid temperature (t C ), the hot fluid temperature (t H ) (step  202 ). Using these inputs and the properties such as specific heat and density of the cold fluid, hot fluid and mixed fluid (at the target temperature), the cold fluid flow controller calculates the cold fluid flow rate (Q C ) according to EQN. 1, where Q T =Q T1  and t T =t T1  (step  204 ). The cold fluid flow controller sets a temperature set point t SP  for the hot fluid flow controller (step  206 ). For example, t SP  can be calculated or set to t T1 .  
         [0040]     When a trigger signal is received (step  208 ), the cold fluid flow controller can begin regulating fluid flow using Q C  as a flow rate set point and issue commands the hot fluid flow controller to regulate flow of the hot fluid (step  210 ). The cold fluid flow controller can adjust the flow of cold fluid according to fluid flow control schemes known in the art, including but not limited to differential control schemes, integral control schemes, proportional integral control schemes, fuzzy logic or proportional integral differential control schemes. If the fluid flow of cold water is greater than the fluid flow set point, cold fluid flow controller can decrease the flow rate (step  212 ), if the fluid flow of cold water is less than the fluid flow set point, the cold fluid flow controller can increase the flow rate, and if the cold fluid flow rate equals the set point (within an acceptable system tolerance) (step  214 ), the cold fluid flow controller can maintain the flow rate (step  216 ). Thus, the cold fluid flow controller can adjust the flow rate of cold fluid based on the target cold fluid flow rate set point Q C .  
         [0041]     Turning to  FIG. 2B , the hot fluid flow controller, on the other hand, can adjust the flow rate of the hot fluid based on the temperature of the mixed fluid (t M1 ) and the mixed fluid set point (t SP ). The temperature of the mixed fluid can be received either directly from a temperature sensor or from the cold fluid flow controller. If t M1  is greater than t SP , the hot fluid flow controller decreases the flow rate of fluid (step  218 ), if t M1  is less than t SP , the hot fluid flow controller increases the flow rate of the hot fluid (step  220 ) and if t M1  is equal to t SP  (within acceptable system tolerances), the hot fluid flow controller maintains the flow rate of hot fluid (step  222 ).  
         [0042]     The steps of  FIGS. 2A and 2B  can be repeated as needed or desired. Moreover, the various steps can be performed in a variety of orders and various steps performed by each flow controller can be performed in parallel.  
         [0043]     While, in the embodiment of  FIGS. 2A and 2B , the cold water flow controller is responsible for determining the set point t SP  for the hot water flow controller, in other embodiments, the hot water flow controller can determine t SP  for itself or provide t SP  to the cold water flow controller so that the cold water flow controller can regulate flow based on t M . In other words, the roles of the hot and cold water flow controllers can be reversed and the steps of  FIG. 2  can be otherwise distributed between the controllers.  
         [0044]     Thus, one embodiment of the present invention can include a first flow controller (e.g. flow controller  104 ), a second flow controller (e.g. flow controller  102 ) and a mixer downstream of the first and second flow controllers. The first flow controller can regulate the flow of a first fluid based on a target flow rate for the first fluid (e.g., Q C ), and the second flow controller can regulate the flow of a second fluid based on a temperature set point and a temperature of the mixed fluid created by the mixer.  
         [0045]     The system of  FIG. 1  can be implemented as a subsystem of a larger mixing system that combines the mixed fluid with additional fluids, such as other chemicals.  FIG. 3  illustrates a solution mixing system  300  that incorporates the subsystem of  FIG. 1 . In the example of  FIG. 3 , solution mixing system  300  provides a concentrated NaCl solution mixing system in which the mixed DIW  118  is combined with NaCl to produce dilute NaCl  302 . In addition to the components discussed in conjunction with  FIG. 1 , solution mixing system  300  includes one or more sources of concentrated NaCl (here illustrated as 1800 parts per million (ppm) NaCl source  304 , 2000 ppm source  306  and 2200 ppm source  308 ). A chemical flow controller  310  controls the flow of concentrated NaCl to a second mixer  312  where the concentrated chemical is mixed with mixed DIW  118 . Mixer  312 , according to one embodiment of the present invention can be a static mixer.  
         [0046]     For the sake of example, cold fluid flow controller  104  can act as a master controller for hot fluid flow controller  102  and chemical flow controller  310 . Cold fluid flow controller  104  receives a target mixed chemical flow rate (Q T2 ) for dilute NaCl  302 , a target mixed chemical ratio for dilute NaCl, a target mixed chemical temperature (t T2 ) for dilute NaCl  302 , t C , and t H . Based on the target mixed chemical flow rate Q T2  and the target mixed chemical ratio, cold fluid controller  104  can determine the target flow rate of DIW (Q T1 ) and flow rate of concentrated NaCl (Q chem ). Assuming that the temperature of the concentrated chemical has a negligible effect on the temperature of dilute NaCl  302 , the target temperature of mixed DIW  118  can be set equal to t T2  (i.e., t T1 =t T2 ). Using t T2 , Q T1  and the input temperatures of the hot and cold DIW, cold fluid flow controller  104  can further determine the target cold DIW flow rate (Q C ) and temperature set point t SP  for hot fluid flow controller  104 . Cold fluid flow controller  104  provides t SP  to hot fluid flow controller  102  and Q chem  to chemical flow controller  310 . Each flow controller can then control the flow of its respective fluid.  
         [0047]      FIGS. 4A-4C  are flow charts illustrating one embodiment of a method for controlling flow of fluids to create a mixed fluid. The method of  FIGS. 4A-4C  can be implemented as computer instructions that are executable by a processor stored on a computer readable medium. For example, embodiments of the present invention can be implemented through programming of one or more OptiChem P1200 LFC flow controllers.  
         [0048]      FIG. 4A  corresponds to the control method implemented at the cold fluid flow controller (e.g., flow controller  104  of  FIG. 3 ),  FIG. 4B  corresponds to the control method implemented at the hot fluid flow controller (e.g., flow controller  102  of  FIG. 3 ) and  FIG. 4C  to the control method implemented at chemical flow controller  310 .  
         [0049]     The cold fluid flow controller receives inputs including the target mixed chemical mix ratio, the target mixed chemical flow rate (Q T2 ), the cold fluid temperature (t C ), the hot fluid temperature (t H ), the target mixed chemical temperature (t T2 ) (step  402 ). Using the target mixed chemical mix ratio and the target mixed chemical flow rate Q T2 , the cold fluid flow controller can determine the target DIW flow rate Q T1  and the flow rate of the concentrated chemical or other fluid (Q chem ) (e.g., NaCl in the example of  FIG. 3 ) (step  406 ). Assuming that the flow of NaCl will have little effect on the overall temperature of the mixed chemical, the cold fluid flow controller can set the target mixed DIW temperature (t T1 ) equal to the target mixed chemical temperature (t T2 ) and determine Q C  according to EQN 1, where Q T =Q T1  (step  408 ). Additionally, the cold fluid flow controller can set t SP =t T1 =t T2  (also shown at  409 ).  
         [0050]     When a trigger signal is received (step  410 ), the cold fluid flow controller can begin regulating fluid flow using Q C  as a flow rate set point, issue commands to the hot fluid flow controller to regulate flow of the hot fluid and issue commands to the chemical flow controller to control flow of the third fluid. The cold fluid flow controller can for adjust the flow of cold fluid according to fluid flow control schemes known in the art, including but not limited to differential control schemes, integral control schemes, proportional integral control schemes, fuzzy logic or proportional integral differential control schemes. If the fluid flow of cold water is greater than the fluid flow set point, cold fluid flow controller can decrease the flow rate (step  412 ), if the fluid flow of cold water is less than the fluid flow set point (step  414 ), the cold fluid flow controller can increase the flow rate, and if the cold fluid flow rate equals the set point (within an acceptable system tolerance), the cold fluid flow controller can maintain the flow rate (step  416 ). Thus, the cold fluid flow controller can adjust the flow rate of cold fluid based on the cold fluid flow rate set point Q C .  
         [0051]     As shown in  FIG. 4B , the hot fluid flow controller can adjust the flow rate of the hot fluid based on the temperature of the mixed fluid (t M1 ) and the mixed fluid set point (t SP ). The temperature of the mixed fluid can be received either directly from a temperature sensor or from the cold fluid flow controller. If t M1  is greater than t SP , the hot fluid flow controller decreases the flow rate of fluid (step  418 ), if t M1  is less than t SP , the hot fluid flow controller increases the flow rate of the hot fluid (step  420 ) and if t M1  is equal to t SP  (within acceptable system tolerances), the hot fluid flow controller maintains the flow rate of hot fluid (step  422 ).  
         [0052]     The chemical flow controller can similarly adjust the flow of the additional fluid (e.g., concentrated NaCl) based on Q chem  as is shown in  FIG. 4C . If the fluid flow of the concentrated chemical (or other fluid) is greater than the Q chem , chemical flow controller can decrease the flow rate (step  428 ), if the fluid flow of the concentrated chemical is less than Q chem  (step  430 ), the cold fluid flow controller can increase the flow rate, and if the concentrated chemical flow rate equals the set point (within an acceptable system tolerance), the chemical flow controller can maintain the flow rate (step  434 ). Thus, the chemical flow controller can adjust the flow rate of concentrated chemical based on the cold fluid flow rate set point Q chem .  
         [0053]     The flow charts of  FIGS. 4A-4C  represent one example embodiment of the present invention. However, it should be understood, that the steps of  FIGS. 4A-4C  can be repeated as needed or desired and can be performed in different orders. Moreover, the steps implemented at each flow controller can be performed in parallel. While, in  FIGS. 4A-4C , the cold water flow controller is responsible for calculating various parameters and asserting set points to the hot water flow controller and chemical flow controller, the step of  FIGS. 4A-4C  can be otherwise distributed to the flow controllers. Additionally, the roles of the hot water and cold water flow controllers can be reversed such that the hot water flow controller controls flow based on a flow rate set point and the cold water flow controller controls flow based on a temperature set point.  
         [0054]     In the embodiment of  FIGS. 3 and 4 A- 4 C, it is assumed that t T2  is not greatly affected by the temperature of the additional fluid added at the second mixer  312 . Thus, it is assumed that the temperature of fluid at the outlet of mixer  312  (t M2 ) is approximately t M1  (i.e., is approximately the temperature of the mixed DIW). According to another embodiment of the present invention, an additional temperature sensor can be used to measure t M2  so that this temperature can be used in flow control.  
         [0055]      FIG. 5  is a diagrammatic representation of one embodiment of a solution mixing system  500  similar to that of  FIG. 3  that adds a conductivity meter  502  and an additional temperature sensor  504  downstream of second mixer  312 . Because the conductivity of a fluid is typically related to the concentration of a fluid, the feedback from conductivity sensor  502  can be used to adjust the concentration of concentrated chemical added at static mixer  312  to achieve a desired conductivity. Additionally, the temperature read by temperature sensor  504  can be used to adjust the flow rates of the hot and cold DIW.  
         [0056]     For the sake of example, cold fluid flow controller  104  can act as a master controller for hot fluid flow controller  102  and chemical flow controller  310 . Initially, cold fluid flow controller  104  receives a target mixed chemical flow rate (Q T2 ), a target mixed chemical ratio, a target mixed chemical temperature (t T2 ), t C , and t H . Based on the target mixed chemical flow rate Q T2  and the target mixed chemical ratio, cold fluid controller  104  can determine the target flow rate of DIW (Q T1 ) and flow rate of concentrated NaCl (Q chem ). Initially, t T1  can be set equal to t T2 . Using Q T1 , t T2 , and the input temperatures of the hot and cold DIW, cold fluid flow controller  104  can further determine the target cold DIW flow rate (Q C ) and temperature set point t SP  for hot fluid flow controller  104 . t SP  can also initially be set equal to t T2 . Cold fluid flow controller  104  provides t SP  to hot fluid flow controller  102  and Q chem  to chemical flow controller  310 . Each flow controller can then control the flow of its respective fluid.  
         [0057]     According to one embodiment, controller  104  can use the temperature of the dilute chemical (t M2 ) to adjust the flow rates of hot and cold DIW. Although control using t M2  can begin immediately, according to other embodiments, cold fluid flow controller  104  can wait a predefined period of time before beginning control using t M2 . This can be done, for example, to allow the flow and temperature of the dilute chemical to settle.  
         [0058]     Cold fluid flow controller  104 , according to one embodiment, can adjust Q C  and t SP  based on the measured temperature of the mixed chemical (t M2 ). For example, given t M2  from temperature sensor  504 , cold fluid flow controller  104  can set the new t SP  equal to:
 
 t   SP(n)   =t   SP(n−1) +( t   T2   −t   M2 )  [EQN. 2]
 
         [0059]     Thus, if t M2  is greater than t T2 , the t SP  is lowered, leading to a decrease in the temperature of DIW, and if t M2  is less than t T2 , t SP  is raised, leading to an increase in the temperature of DIW. Cold fluid flow controller  104  can further determine a new target flow rate for the cold DIW (i.e., a new Q C ) using the t SP  calculated in EQN 2 for t T  of EQN 1. As described above, cold fluid flow controller  104  can regulate flow according to Q C  and hot fluid flow controller  102  can regulate flow according to t SP  and t M1 .  
         [0060]      FIGS. 6A-6C  are flow charts illustrating one embodiment of a method for controlling flow of fluids to create a mixed fluid. The method of  FIGS. 6A-6C  can be implemented as computer instructions that are executable by a processor stored on a computer readable medium. For example, embodiments of the present invention can be implemented through programming of one or more OptiChem P1200 LFC flow controllers.  
         [0061]      FIG. 6A  corresponds to the control method implemented at the cold fluid flow controller (e.g., flow controller  104  of  FIG. 5 ),  FIG. 6B  corresponds to the control method implemented at the hot fluid flow controller (e.g., flow controller  102  of  FIG. 5 ) and  FIG. 6C  corresponds to the control method implemented at chemical flow controller  310 .  
         [0062]     The cold fluid flow controller receives inputs including target mixed chemical mix ratio, the target mixed chemical flow rate (Q T2 ), the cold fluid temperature (t C ), the hot fluid temperature (t H ), the target mixed chemical temperature (t T2 ) (step  602 ). Using the target mixed chemical mix ratio and the target mixed chemical flow rate Q T2 , the cold fluid flow controller can determine the target DIW flow rate Q T1  and the f low rate of the concentrated chemical or other fluid (Q chem ) (e.g., NaCl in the example of  FIG. 5 ) (step  606 ). Flow controller  102  can initially act as if the flow of NaCl will have little effect on the temperature of t T2 . Therefore, the cold fluid flow controller can set t T =t T2  and determine Q C  according to EQN 1, where Q T =Q T1  and t T =t T2  (step  608 ). Additionally, the cold fluid flow controller can set t SP =t T  (also shown at  609 ).  
         [0063]     When a trigger signal is received (step  610 ), the cold fluid flow controller can begin regulating fluid flow using Q C  as a flow rate set point, issue commands the hot fluid flow controller to regulate flow of the hot fluid and issue commands to the chemical flow controller to control flow of the third fluid. The cold fluid flow controller can for adjust the flow of cold fluid according to fluid flow control schemes known in the art, including but not limited to differential control schemes, integral control schemes, proportional integral control schemes, proportional integral differential, or fuzzy logic control schemes. If the fluid flow of cold water is greater than the fluid flow set point, cold fluid flow controller can decrease the flow rate (step  616 ), if the fluid flow of cold water is less than the fluid flow set point (step  618 ), the cold fluid flow controller can increase the flow rate, and if the cold fluid flow rate equals the set point (within an acceptable system tolerance), the cold fluid flow controller can maintain the flow rate (step  620 ). Thus, the cold fluid flow controller can adjust the flow rate of cold fluid based on the cold fluid flow rate set point Q C .  
         [0064]     The cold fluid flow controller can also receive the temperature of the mixed chemical from a temperature sensor downstream of the second mixer (e.g., can receive t M2  from temperature sensor  504  of  FIG. 5 ) (step  622 ). Using t M2 , the cold fluid flow controller can calculate a new Q C  and t M2  as, for example, described in conjunction with  FIG. 5  (step  638 ). Cold fluid flow controller can then perform steps  618 - 620  using the new Q C  and pass the new t SP  to the hot fluid flow controller. According to one embodiment, Q C  and t SP  can be continually updated as t M2  changes.  
         [0065]     As shown in  FIG. 6B  the hot fluid flow controller, can adjust the flow rate of the hot fluid based on the temperature of the mixed fluid (t M1 ) and the mixed fluid set point (t SP ). The temperature of the mixed fluid can be received either directly from a temperature sensor or from the cold fluid flow controller. Hot water flow controller  104  receives the initial temperature set point t SP  (step  623 ). If t M1  is greater than t SP , the hot fluid flow controller decreases the flow rate of fluid (step  624 ), if t M1  is less than t SP , the hot fluid flow controller increases the flow rate of the hot fluid (step  626 ) and if t M1  is equal to t SP  (within acceptable system tolerances), the hot fluid flow controller maintains the flow rate of hot fluid (step  628 ). The hot fluid flow controller can receive the new temperature set point at step  629  and perform steps  624 - 628  accordingly.  
         [0066]     The chemical flow controller can similarly adjust the flow of the additional fluid (e.g., concentrated NaCl) based on Q chem . If the fluid flow of the concentrated chemical (or other fluid) is greater than the Q chem , chemical flow controller can decrease the flow rate (step  630 ), if the fluid flow of the concentrated chemical is less than Q chem  (step  632 ), the cold fluid flow controller can increase the flow rate, and if the concentrated chemical flow rate equals the set point (within an acceptable system tolerance), the chemical flow controller can maintain the flow rate (step  634 ). Thus, the chemical flow controller can adjust the flow rate of concentrated chemical based on the cold fluid flow rate set point Q chem .  
         [0067]     Additionally, the chemical flow controller can receive a measurement of conductivity of the mixed chemical (step  640 ). Using the conductivity, the flow controller can adjust the concentration of chemical added at the second mixer. If the conductivity indicates that the mixed chemical is too concentrated, the flow controller can decrease the concentration of concentrated chemical (step  642 ). If the conductivity sensor indicates that the mixed chemical is too dilute, the flow controller can increase the concentration of the concentrated chemical added to the DIW. Otherwise, the concentration can be unchanged (step  646 ).  
         [0068]     The flow charts of  FIGS. 6A-6C  represent one example embodiment of the present invention. However, it should be understood, that the steps of  FIGS. 6A-6C  can be repeated as needed or desired and can be performed in different orders. Moreover, the steps implemented at each flow controller can be performed in parallel. While, in  FIGS. 6A-6C , the cold water flow controller is responsible for calculating various parameters and asserting set points to the hot water flow controller and chemical flow controller, the steps of  FIGS. 6A-6C  can be otherwise distributed to the flow controllers. Additionally, the roles of the hot water and cold water flow controllers can be reversed such that the hot water flow controller controls flow based on a flow rate set point and the cold water flow controller controls flow based on a temperature set point.  
         [0069]     As discussed above, the various flow controllers can control the flow of fluids to the mixers, the mixers (e.g., mixer  106  and mixer  312 ), which can optionally be static mixers.  FIGS. 7A-7F  provide diagrammatic representations of one embodiment of a static mixer assembly  700  and its components. Referring to  FIG. 7A , static mixer assembly  700  includes a mixer housing  702 , an inlet assembly  704  and an outlet assembly  706 . Inlet assembly  704  includes two inlets, inlet  708  and inlet  710 . These inlets can be coupled to fluid supply lines that lead from upstream flow controllers. For example, inlet  708  can receive hot DIW from hot DIW flow controller  102  and inlet  710  can receive cold DIW from cold DIW flow controller  104 . In the example shown in  FIG. 7A , inlet assembly  704  has male threaded sections  712  and  714  to connect to inlet supply lines. Similarly, outlet assembly  706  has male threaded section  716  to connect to an outlet line.  
         [0070]      FIG. 7B  is a partial cutaway of mixer assembly  700  and illustrates a flow path  718  defined through mixer housing  702  from inlet assembly  704  to outlet assembly  706 . Thus, fluids entering inlet  708  and inlet  710  of inlet assembly  704  exit a common outlet.  FIG. 7B  further illustrates that inlet assembly  704  can include a male threaded portion  719  and outlet assembly  706  can include a male threaded portion  720  to couple to mixer housing  702 , which has corresponding female threaded portions.  
         [0071]      FIG. 7C  illustrates another partial cutaway of mixer assembly  700 . As shown in  FIG. 7C , mixer assembly  700 , according to one embodiment of the present invention includes a mixer disk  722  that acts as a static mixer. In the embodiment of  FIG. 7C , mixer disk  722  is located in mixer housing  702  at the outlet side of inlet assembly  704 . Mixer disk  722  can include a seating flange  724  that rests in a corresponding annular ring of housing assembly  702 . Seating flange  724 , working in concert with the annular ring as a tongue and groove fitting, can ensure proper seating of mixer disk  722  in mixer housing  702 . Additionally, mixer disk  722  can include an annular ring  726  on its upstream side that receives a flange on the outlet side of inlet assembly  704 . This also aids in proper seating of mixer disk  722 .  
         [0072]     By way of example, but not limitation, inlet assembly  704  and outlet assembly  706  are configured to connect to ⅜ inch O.D. tubing with a 0.25 inch bore and flow path  718  has a 0.21 inch diameter. Moreover, the various components of mixer assembly  700 , according to one embodiment, can be machined or molded from Teflon or modified Teflon.  
         [0073]      FIG. 7D  is a diagrammatic representation of one embodiment of mixer disk  722  showing one embodiment of the upstream side. Mixer disk  722 , according to one embodiment of the present invention, includes an outer section  728  defined by an outer surface  729  at an outer circumference and an inner surface  730  at an inner circumference  731 . Additionally, outer section  728  can include an annular ring  726  that receives, as discussed above, a flange on the outlet side of inlet assembly  704  to aid in seating.  
         [0074]     In the embodiment of  FIG. 7D , an inner flange  732  projects inwardly from inner surface  730  with an inner flange surface  733  that defines a flow passage. Two radially opposed mixing tabs (tab  736  and  738 ) further project inwardly towards each other. According to the preferred embodiment, mixing tab  736  and  738  do not touch, but have a small gap between them to leave the center of the flow passage unobstructed. Mixing tab  736  and mixing tab  738  can have downstream surfaces extending approximately normal to inner flange surface  733  and inclined upstream surfaces such that the mixing tabs are thinner near the center of the flow passage and wider proximate to inner flange  732 . According to one embodiment, the upstream surfaces of mixing tabs  736  and  738  are inclined approximately fifteen degrees.  
         [0075]     Mixer disk  722  can further include an alignment notch  740  to align mixer disk  722  in mixer assembly housing  702 . Alignment notch  740  can mate with a corresponding protrusion in mixer assembly housing  702  to align mixer disk  722  to have a particular orientation. For example, mixer disk  722  can be aligned such that mixing tabs are oriented in particular direction.  
         [0076]      FIG. 7E  is a diagrammatic representation of mixer disk  722  from an upstream view. By way of example, but not limitation, the outer diameter of outer section  728  can be 0.55 inches, and the inner diameter 0.21 inches. The inner diameter of inner flange  732  can further be 0.166 inches. Each of mixing tabs  736  and  738  can extend inwardly 0.074 from inner flange  732  with a gap of 0.018 inches between the mixing tabs. Again, by way of example, annular groove  726  can have an outer diameter of 0.45 inches and a thickness of 0.029 inches. It should be noted that these dimensions are provided by way of example and not limitation and larger or smaller mixing disks can be used. Additionally, the various radii or other example dimensions can be differently proportioned relative to each other.  
         [0077]      FIG. 7F  is a section view of one embodiment mixer disk  722  along line AA of  FIG. 7E . In addition to the features discussed in conjunction with  FIG. 7D ,  FIG. 7F  illustrates seating flange  724 . In this embodiment, seating flange  724  is an annular ring projecting from the downstream side of mixer disk  722 . It can also be noted from  FIG. 7F  that tabs  736  and  738  can be wedge shaped with the upstream surface of each tab angling 15 degrees inward as it approaches the center of mixer disk  722 .  
         [0078]     The downstream surface, on the other hand, remains perpendicular to the flow passage. The tabs can have other shapes and there can be more than two tabs, or a single tab. Additionally, the dimensions and angles shown in  FIG. 7F  are provided by way of example, but not limitation.  
         [0079]      FIGS. 8A-8C  provide diagrammatic representations of another embodiment of a mixer assembly. Referring to  FIG. 8A , static mixer assembly  800  includes a mixer housing  802 , three inlet assemblies  804 ,  806  and  808  an outlet assembly  810 . Each of the inlet assemblies can include an inlet connected by a supply line to supply a fluid. Using the example of the mixing system of  FIG. 3 , inlet assembly  804  includes an inlet through which the mixed fluid (e.g., mixed DIW) can supplied (e.g., from mixer  106  of  FIG. 3 ) while inlet assemblies  806  and  808  include inlets through which concentrated chemical can be provided by a chemical flow controller (e.g., chemical flow controller  310  of  FIG. 3 ). In the example shown in  FIG. 8A , inlet assemblies  804 ,  806  and  808  have male threaded sections  812 ,  814  and  816 , respectively, to connect to inlet supply lines. Similarly, outlet assembly  810  has male threaded section  818  to connect to an outlet line.  
         [0080]      FIG. 8B  is a partial cutaway of mixer assembly  800  and illustrates a flow path  820  defined through mixer housing  802  from inlet assembly  804  to outlet assembly  810 . Additionally,  FIG. 8B  illustrates fluid flow paths  822  and  824  through inlet assemblies  806  and  808 , respectively, which join with flow path  820 . Thus, fluids entering inlet assembly  804 , inlet assembly  806  and inlet assembly  808  exit a common outlet.  FIG. 78  further illustrates that inlet assembly  804  can include male threaded portion  824 , inlet assembly  806  can include male threaded portion  826 , inlet assembly  808  includes male threaded portion  828  and outlet assembly  810  can include a male threaded portion  830  to couple to mixer housing  802 , which has corresponding female threaded portions.  
         [0081]      FIG. 8C  illustrates a cross sectional view of one embodiment of mixer assembly  800 . As shown in  FIG. 8C , mixer assembly  800 , according to one embodiment of the present invention, includes a mixer disk  832  that acts as a static mixer. In the embodiment of  FIG. 8C , mixer disk  832  is located in mixer housing  802  at the outlet side of inlet assembly  804 . Mixer disk  832  can include a seating flange  834  that rests in a corresponding annular ring of housing assembly  802 . Seating flange  834 , working in concert with the annular ring as a tongue and groove fitting, can ensure proper seating of mixer disk  832  in mixer housing  802 . Additionally, mixer disk  832  can include an annular ring  836  that receives a flange on the outlet side of inlet assembly  804 . This also aids in proper seating of mixer disk  832 .  
         [0082]      FIG. 8C  also illustrates that flow passages  822  and  824  intersect with flow passage  820  downstream of mixer disk  832 . Consequently, in a mixing system such as that depicted in  FIG. 3 , the concentrated chemical is introduced downstream of mixing disk  822 .  
         [0083]     By way of example, but not limitation, inlet assembly  804 , inlet assembly  806 , inlet assembly  808  and outlet assembly  810  are configured to connect to ⅜ inch O.D. tubing with a 0.25 inch bore. By way of example, but not limitation, flow path  218  has a 0.21 inch diameter. The various components of mixer assembly  800 , according to one embodiment, can be machined or molded from Teflon or modified Teflon. Mixer disk  822  can be similar or identical to mixer disk  722  of  FIGS. 7D-7F . Mixing disk  822  can be aligned (e.g. using the alignment notch) such that the tabs of mixing disk  822  are aligned over flow passage  822  and flow passage  824 .  
         [0084]     As described above, embodiments of the present invention can provide a fluid mixing system that utilizes various flow controllers (e.g., hot DIW controller  102 , cold DIW controller  104  and chemical flow controller  310 ). According to various embodiments, one of the flow controllers can act as a master controller that communicates set points to the other flow controllers. Thus, the master flow controller is preferably capable of asserting multiple set points.  
         [0085]     Many existing flow controllers receive set points as analog voltages/current. Typically, this requires the use of multiple analog sources to provide set points to different flow controllers. However, a particular flow controller may only have one or a limited number of analog ports available. This limits the number of slave flow controllers to which a particular master flow controller can assert set points. Embodiments of the present invention reduce or eliminate the deficiencies associated with having a limited number of analog ports by providing for multiplexing of analog set points on a particular analog communications link.  
         [0086]      FIG. 9  is a diagrammatic representation of one embodiment of a system  900  for multiplexing analog set points. System  900  includes an analog signal source  902  connected to multiple slave devices  904   a - 904   d  via an analog communications link  906  and one or more parallel digital communications links  908 . Analog signal source  902  can be a flow controller, such as an OptiChem P1200 produced by Mykrolis, Inc. of Billerica, Mass. (now part of Entegris Corporation of Chaska, Minn.). Similarly, devices  904   a - 904   d  can also be OptiChem P1200 flow controllers. In other words, one flow controller, acting as analog signal source  902  can act as a master device to other flow controllers. It should be noted, however, that analog signal source  902  can be any device capable of asserting an analog set point and devices  904   a - 904   d  can be any devices capable of receiving analog set points.  
         [0087]     Analog signal source  902  outputs an analog signal including set points for multiple slave devices on analog communications link  906 . Digital communications links  908   a - 908   d  can carry set point indicator signals to each of slave devices  904   a - 904   d.  It should be noted that the digital communications links can be separate busses or the same bus arbitrated to send a digital signal to a particular slave device  904 . A set point indicator signal for a particular slave device indicates that the analog signal is indicating the set point for that slave device. When a particular slave device  904  receives an indication that the analog signal is specifying the set point for that device, the particular slave device  904  can read its set point from the analog signal. Using the set point indicator signals to indicate when set points for particular devices are being asserted on an analog line allows multiple analog set points to be multiplexed on a single analog bus  906 .  
         [0088]     In  FIG. 9 , the analog set point signal and set point indicator signals are illustrated as coming from the same master device. However, in other embodiments of the present invention, the analog set point signal and set point indicator signals can be generated at distributed devices.  
         [0089]      FIG. 10  illustrates one embodiment of a an analog set point signal  1000  asserted by analog signal source  902 , a set point indicator  1002  signal for slave device  904   a  a set point indicator signal  1004  for slave device  904   b , a set point indicator signal  1006  for slave device  904   c  and a set point indicator signal  1008  for slave device  904   d.  According to the embodiment illustrated in  FIG. 10 , analog set point signal  1000  can have voltages/current between 0% and 100% of a full scale value, whereas the set point indicator signals are either high or low (e.g., cycling between +/−3.3 volts or other voltage values or other values indicating a setpoint).  
         [0090]     In the example of  FIG. 10 , four analog set points are multiplexed into analog signal  1000 . For time period t 1  the set point is 45% of full scale; for time period t 2 , the set point is 62% of full scale; for time period t 3 , the set point is 30% of full scale; and for time period t 4 , the set point is 78% of full scale.  
         [0091]     The analog set point values may have different meanings for the various slave devices. For example, the analog set point may correspond to a pressure at slave device  904   a , but a pump motor speed at slave device  904   b . Thus, the analog set point signal can multiplex analog set points for a variety of purposes.  
         [0092]     During at least part of time period t 1 , set point indicator signal  1002  changes states from high to low (shown at  1010 ) indicating that slave device  904   a  should use the 45% of full scale value as its set point. Slave device  904   a  can continue to use this set point value until the set point indicator signal indicates that it should read a new set point from the analog set point signal  1000 . Thus, slave device  904   a  can continue to use the 45% of full scale set point even though the value of the analog signal is changing.  
         [0093]     Similarly, set point indicator signal  1004  indicates that slave device  904   b  should use the 62% of full scale as its set point (shown at  1012 ), set point indicator signal  1006  indicates that slave device  904   c  should use the 30% of full scale as its set point (shown at  1014 ) and set point indicator signal  1008  indicates that slave device  904   d  should use the 78% of full scale as its set point (shown at  1016 ).  
         [0094]     The signal timings provided in  FIG. 10  are provided by way of example and any suitable scheme for indicating to a slave device when the analog signal is carrying the set point for that device can be utilized. For example, the set point indicator signal can change states (e.g., from low to high, from high to low or undergo other state change) when the slave device should begin reading its set point from the analog set point signal and change states again when the slave device should stop reading its set point from the analog set point signal. Additionally, the set point indicator can be sent to the slave devices in a variety of manners, including as part of a data stream, an interrupt or in another manner.  
         [0095]     According to another embodiment of the present invention, the set point indicator signal can be asserted on multiple digital lines.  FIG. 11  is a diagrammatic representation of one embodiment of a system  1100  for multiplexing analog set points. System  1100  includes an analog signal source  1102  connected to multiple slave devices  1104   a - 1104   d  via an analog communications link  1106  and a digital bus  1107 . Digital bus  1107  is connected to slave devices  1104   a - 1104   d  at  1108   a - 1108   d  respectively. Digital bus  1107  can include any number of lines for carrying signals to slave devices  1104   a - 1104   d.  In the example of  FIG. 11 , digital bus has three signaling lines. Analog signal source  1102  can be a flow controller, such as an OptiChem P1200 produced by Mykrolis, Inc. of Billerica, Mass. (now part of Entegris Corporation of Chaska, Minn.). Similarly, devices  1104   a - 1104   d  can also be OptiChem P1200 flow controllers. In other words, one flow controller, acting as analog signal source  1102  can act as a master device to other flow controllers. It should be noted, however, that analog signal source  1102  can be any device capable of asserting an analog set point and devices  1104   a - 1104   d  can be any devices capable of receiving analog set points.  
         [0096]     Analog signal source  1102  outputs an analog signal including set points for multiple slave devices on analog communications link  1106 . Digital bus  1107  can carry set point indicator signals to each of slave devices  1104   a - 1104   d.  A set point indicator signal for a particular slave device indicates that the analog signal is indicating the set point for that slave device. The set point indicator signal for a particular slave device  1104  can be asserted as multiple bits on bus  1107 . For example, the set point indicator for slave device  1104   d  can be bits asserted on the second and third signaling lines of bus  1107  (e.g., 011). When a particular slave device  1104  receives an indication that the analog signal is specifying the set point for that device, the particular slave device  1104  can read its set point from the analog signal. Implementing a binary weighted system for each of the digital select line extends the capabilities of the master without increasing the number of digital setpoint indicator lines.  
         [0097]     In  FIG. 11 , the analog set point signal and set point indicator signals are illustrated as coming from the same master device. However, in other embodiments of the present invention, the analog set point signal and set point indicator signals can be generated at distributed devices.  
         [0098]      FIG. 12  illustrates one embodiment of a an analog set point signal  1200  asserted by analog signal source  1102 , and digital signals for providing setpoint indicators. According to the embodiment illustrated in  FIG. 12 , analog set point signal  1200  can have voltages/current between 0% and 100% of a full scale value, whereas the set point indicator signals are either high or low (e.g., cycling between +/−3.3 volts or other voltage values or other values indicating a setpoint).  
         [0099]     In the example of  FIG. 12 , four analog set points are multiplexed into analog signal  1300 . For time period t 1  the set point is 45% of full scale; for time period t 2 , the set point is 62% of full scale; for time period t 3 , the set point is 30% of full scale; and for time period t 4 , the set point is 78% of full scale.  
         [0100]     The analog set point values may have different meanings for the various slave devices. For example, the analog set point may correspond to a pressure at slave device  1104   a , but a pump motor speed at slave device  1104   b . Thus, the analog set point signal can multiplex analog set points for a variety of purposes.  
         [0101]     During at least part of time period t 1 , set point signal  1202  changes states from high to low (shown at  1210 ) indicating that slave device  1104   a  should use the 45% of full scale value as its set point. Slave device  1104   a  can continue to use this set point value until the set point indicator signal indicates that it should read a new set point from the analog set point signal  1200 . Thus, slave device  1104   a  can continue to use the 45% of full scale set point even though the value of the analog signal is changing.  
         [0102]     Similarly, signal  1204  indicates that slave device  1104   b  should use the 62% of full scale as its set point (shown at  1212 ), signal  1206  indicates that slave device  1104   c  should use the 30% of full scale as its set point (shown at  1314 ). In time t 4 , signals  1204  and  1206  assert a bit (shown at  1216  and  1218 ), indicating that slave device  1104   d  should use the 78% of full scale as its set point (i.e., multiple digital lines are used to send the setpoint indicator to slave device  1104   d ). Thus, three set point indicator lines are used to indicate setpoint to four slave devices. Using a binary scheme up to 7 slave devices can be supported (2 n −1, where n is the number of setpoint indicator lines) with one signal state reserved for the case in which no setpoint is being asserted for a device.  
         [0103]     The signal timings provided in  FIG. 12  are provided by way of example and any suitable scheme for indicating to a slave device when the analog signal is carrying the set point for that device can be utilized. For example, the set point indicator signal can change states (e.g., from low to high, from high to low or undergo other state change) when the slave device should begin reading its set point from the analog set point signal and change states again when the slave device should stop reading its set point from the analog set point signal. Additionally, the set point indicator can be sent to the slave devices in a variety of manners, including as part of a data stream, an interrupt or in another manner.  
         [0104]      FIG. 13  is a flow chart illustrating one embodiment of a method for multiplexing analog set points. The flow chart is divided into two sections for the master and slave device. The methodology of  FIG. 13  can be implemented, for example, by execution of computer instructions at the master, slave or other devices.  
         [0105]     According to one embodiment, an analog signal source generates an analog signal representing multiple set points (step  1302 ). Put another way, multiple analog set points are multiplexed in the analog signal. The master device communicates the analog signal to the slave devices. When the set point for a particular slave device is being transmitted via the analog signal, the master device can send a set point indicator to that slave device (step  1304 ). For example, the master device can use a signal on a digital bus (e.g., by changing the state of a line or lines on the bus) to indicate to a particular slave device that its set point is being asserted on the analog line. The routine can continue until a predefined event occurs to end the routine.  
         [0106]     The slave device can receive the analog set point signal (step  1306 ). When the slave device receives a set point indicator indicating that the analog set point signal is asserting that slave device&#39;s set point (e.g., as determined at  1308 ), the slave device can save the value of the analog set point signal and store the signal as its set point (step  1310 ). This routine can continue until a predefined event occurs to end the routine. Additionally, the steps of  FIG. 13  can be repeated as needed or desired.  
         [0107]     While the present invention has been described with reference to particular embodiments, it should be understood that the embodiments are illustrative and that the scope of the invention is not limited to these embodiments. Many variations, modifications, additions and improvements to the embodiments described above are possible. It is contemplated that these variations, modifications, additions and improvements fall within the scope of the invention as detailed in the following claims.