Abstract:
By monitoring the output voltage of a mass flow controller (MFC) there is no way to detect when a MFC is starting to degrade (or beginning to fail) as long as the MFC output voltage is driven to match the MFC setpoint voltage. Only when the MFC actually fails, and the MFC output voltage is unable to be driven to match the MFC setpoint voltage, is the failure detectable. A predictive failure monitoring system for a mass flow controller (MFC) is disclosed which monitors the “valve voltage” of the MFC as well as the MFC output voltage. By knowing the normal relationship between the MFC setpoint voltage and the respective valve voltage, then degradation or other changes may be noticed before the MFC actually fails, and importantly, before production material is ruined by the failing MFC. Such a real-time monitoring capability may be transparently implemented as an add-on module between the MFC and the system controller for the MFC. The add-on module may be advantageously connected between the MFC card edge connector and the system card edge socket which is normally connected to the MFC card edge connector. The module monitors, in real time, the performance of the MFC by measuring the valve voltage on the valve test point. If determined to be outside of various user-settable limits, then warnings and relay closures may be done. The module can also automatically characterize the MFC operation when known to be proper, then go into a “run” mode to monitor operation and provide indications when operation falls outside of user-settable limits.

Description:
BACKGROUND OF THE INVENTION 
     Mass Flow Controllers (MFC) are used extensively in modern semiconductor manufacturing to control the flow of various gases into a wide variety of process equipment, such as etchers, deposition reactors, implanters, etc. Many individual pieces of equipment have more than one such MFC, with six to eight MFCs per major piece of semiconductor manufacturing equipment not being uncommon. 
     A representative MFC  100  is shown in three orthogonal views in FIGS. 1A,  1 B, and  1 C. A source of gas is connected to and received at an input port  102  and a controlled flow is delivered to an output port  104  and to downstream equipment attached thereto. A housing  106  contains an electronic module for controlling the MFC which is connected to a system controller (not shown) by way of a cable attached to an MFC card edge connector  108 . 
     To more fully appreciate the invention, a brief description of the inner workings of a typical MFC is warranted. Referring now to FIG. 2, the representative MFC  100  includes four distinct subsystems. A mass flow sensor  120  produces an electrical signal proportional to the mass of the gas flowing therethrough. A control valve  122  acts as a variable orifice to control the total gas flowing through the MFC  100 . A flow bypass  124  diverts a small portion of the total gas flowing into the MFC  100  into and through the mass flow sensor  120 . Lastly, a printed wiring board  126  contains various circuits for controlling the MFC  100 , and typically include a sensor bridge and amplifier circuit, a control circuit, and an RC network, as described more fully below. 
     Referring now to FIG. 3, the basic operation of the MFC  100  is described. The mass flow sensor  120  is illustrated as a “capillary tube” thermal mass flow sensor which is designed to measure the mass of gas flowing through a thin stainless steel capillary sensor tube  131 . Two temperature sensing wire windings  130 ,  132  are attached around the sensor tube  131 . One winding is located on the upstream side of the sensor tube  131  and the other winding is located on the downstream side of the sensor tube  131 . The wire forming these windings  130 ,  132  is resistance thermal detection (RTD) type wire, which means the resistance of such wire is a function of the temperature of the wire. The sensor tube is installed in a protective cover and is usually enclosed in heat insulating material. An equal amount of heat is produced in both sensor windings either directly by a constant current source or by using a separate heater wire winding (not shown) between the upstream sensor winding  130  and the downstream sensor winding  132 . 
     With no gas flowing through the sensor  120 , both the upstream sensor winding  130  and the downstream sensor winding  132  are at the same temperature, and consequently, they both have the same resistance. Since each winding  130 ,  132  has the same resistance, and the same current flows through each winding, the voltage drop across each winding  130 ,  132  is the same. The voltage drop across each winding  130 ,  132  is compared by a sensor bridge and amplifier circuit  134  to produce an MFC output voltage conveyed on sensor output terminal  140 . 
     With 50% gas flow through the mass flow sensor  120 , gas at room temperature flows through the sensor tube  131  and heat from the upstream sensor winding  130  is transferred to the gas molecules. This reduces the temperature of the upstream sensor winding  130 , and increases the temperature of the gas. As this hotter gas flows past the downstream sensor winding  132  it transfers less heat away from the downstream sensor winding  132 . This difference in temperature between the upstream sensor winding  130  and the downstream sensor winding  132  results in a difference in resistance between the two windings  130 ,  132 , which then results in a difference in voltage across the two windings  130 ,  132 . This voltage difference is amplified and linearized by sensor bridge and amplifier circuit  134  to become the MFC output voltage at sensor output terminal  140 . This output voltage is an indirect result of gas molecules flowing through the mass flow sensor  120 . 
     In other words, the difference in temperature between the upstream sensor winding  130  and the downstream sensor winding  132  is sensed as a small (millivolts) non-zero voltage by the sensor bridge and amplifier circuit  134 . This small voltage is amplified to a typical level of several volts and linearized to provide a 0 to 5 volt DC output voltage signal (for many commercial MFCs) which is proportional to the mass of the gas flowing through the mass flow sensor  120 . If the ratio of gas flowing through the mass flow sensor  120  and through the flow bypass  124  is correct, the output signal is proportional to the mass of the gas flowing through the MFC  100  from the input port  102  to the output port  104 . 
     A control circuit  136  compares the output voltage signal produced by the sensor bridge and amplifier circuit  134  against an externally supplied setpoint signal conveyed on terminal  142 . The setpoint signal is usually a 0 to 5 volt DC signal and corresponds to the actual flow desired through the MFC. The control circuit  136  drives a valve control transistor  138  which positions the control valve  122  in such a manner as to eliminate any difference between the setpoint signal and the output signal. If the actual flow (represented by the MFC output voltage) is less than the desired flow (as represented by the MFC setpoint voltage), the control circuit  136  biases the valve control transistor  138  in such a manner as to open the control valve  122  to allow more gas flow through the control valve  122 , and hence through the MFC  100 . As more gas flows through the MFC  100 , proportionally more gas flows through the mass flow sensor  120  causing the MFC output voltage to increase. This reduces the difference between the MFC setpoint voltage and the MFC output voltage. 
     The electrical schematic diagram of a particular manufacturer&#39;s MFC is shown in FIG.  4 . Shown is the schematic for a model FC-2950M mass flow controller/flowmeter available from Tylan General, Inc., located in San Diego and Torrance, Calif. The upstream sensor winding  130  and downstream sensor winding  132  are shown as resistors which connect into a bridge circuit  154 , the outputs of which are then amplified by an amplifier  156  to produce the MFC output voltage at terminal  140 . Various RC feedback circuitry within the amplifier  156  serve to stabilize the operation of the amplifier  156 . The bridge circuit  154 , amplifier  156 , and other feedback and reference circuits shown form the sensor bridge and amplifier circuit  134  described previously. The control circuit  136  receives the MFC output voltage and compares it to the MFC setpoint voltage on terminal  142  to control the valve control transistor  138 , whose output terminal  150  is connected to the control valve  122  (modeled on the schematic as a resistor connected to terminals V 1  and V 2 ) through a current-limiting resistor  158 . Each of the signals shown on the left side of FIG. 4 are usually available at the MFC card edge connector  108 . 
     As described above, the closed-loop feedback operation of the control circuit  136  causes the MFC output voltage on terminal  140  to be driven to match the MFC setpoint voltage on terminal  142 . The “valve voltage” on terminal  150  is adjusted to whatever voltage is required to adjust the gas flow in order to cause the MFC output voltage to match the MFC setpoint voltage. A system controller which is connected to the MFC card edge connector  108  presents to the MFC  100  the desired MFC setpoint voltage, and then monitors the MFC output voltage produced by the MFC  100  in response thereto. 
     SUMMARY OF THE INVENTION 
     The closed loop nature of the MFC  100  results in the MFC output voltage always matching, if at all possible, the MFC setpoint voltage. Unfortunately, subtle changes in the MFC, or in the system around the MFC, are undetectable by the system controller. For example, if the control valve  122  ages and requires a higher voltage to allow a certain amount of gas flow than it previously required, the system controller is unable to notice this degradation in the MFC. Similarly, if a stoppage forms in the piping downstream of the MFC, and the control valve  122  must open a little more to allow a certain gas flow than without the stoppage, the system controller is unable to notice this degradation in the system. 
     By monitoring the MFC output voltage there is no way to detect when a MFC is starting to degrade (or beginning to fail) as long as the MFC output voltage is driven to match the MFC setpoint voltage. Only when the MFC actually fails, and the MFC output voltage is unable to be driven to match the MFC setpoint voltage, is the failure detectable. However, by monitoring the “valve voltage” (e.g., the actual voltage across the control valve  122 , or equivalently a voltage on a node such as node  150  in FIG. 4 which is proportional to the actual valve voltage, or even a current which is representative of the control valve operation) and not just monitoring the MFC output voltage, and by knowing the normal relationship between the MFC setpoint voltage and the respective valve voltage, then degradation or other changes may be noticed before the MFC actually fails, and importantly, before production material is ruined by the failing MFC. 
     Such a real-time monitoring capability may be transparently implemented using an add-on module between the MFC and the system controller. More specifically, the add-on module may be connected between the MFC card edge connector  108  and the system card edge socket which is normally connected to the MFC card edge connector  108 . The module monitors, in real time, the performance of the MFC. If the MFC performance is outside of user limits, various warnings and relay closures may be done. The module can also automatically characterize the MFC operation, then go into a warning/watch mode. The module can monitor the MFC valve voltage on the valve test point, and optionally can also send data to a host computer. 
     This module monitors a closed loop system which heretofore gave no warning of degradation or impending failure. It also eliminates the need for preventive maintenance (PM) which pulls the MFC to check calibration, eliminates scrap wafers due to MFC drift or failure, except in the catastrophic failure mode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. 
     FIGS. 1A,  1 B, and  1 C, labeled prior art, are orthogonal views of a mass flow controller typically used within the semiconductor industry. 
     FIG. 2, labeled prior art, is a two-dimensional drawing depicting the major physical components of a mass flow controller. 
     FIG. 3, labeled prior art, is a two-dimensional drawing depicting the major electrical components of a mass flow controller. 
     FIG. 4, labeled prior art, is an electrical schematic of a representative mass flow controller which is generally available. 
     FIG. 5 is a diagram indicating a predictive failure monitoring system, in accordance with one embodiment of the present invention, and particularly indicating the manner of its connection between a mass flow controller and a system controller which is otherwise normally connected to the MFC. 
     FIG. 6 is a block diagram indicating the major components of the predictive failure monitoring system shown in FIG.  5 . 
     FIG. 7 is a flowchart depicting the major operating blocks of the exemplary predictive failure monitoring system. 
     FIG. 8 is a flowchart depicting the operation of the exemplary predictive failure monitoring system when using equation-based data. 
     FIG. 9 is a flowchart depicting the operation of the exemplary predictive failure monitoring system when using table-based data. 
     FIG. 10 is a flowchart depicting the operation of the exemplary predictive failure monitoring system to create or build a table. 
     FIG. 11 is a flowchart depicting the operation of the exemplary predictive failure monitoring system to collect data ad hoc (the “watch mode”). 
     FIG. 12 is a flowchart depicting the operation of the exemplary predictive failure monitoring system to dump the data table. 
     FIG. 13 is a diagram depicting the operation of an optional LCD display which may be attached to the exemplary predictive failure monitoring system. 
     FIG. 14 is an electrical schematic diagram of an implementation of the exemplary predictive failure monitoring system. 
     FIGS. 15A and 15B are electrical component placement diagrams showing two implementations of the electrical schematic shown in FIG.  14 . 
     The use of the same reference symbols in different drawings indicates similar or identical items. 
    
    
     DETAILED DESCRIPTION 
     FIG. 5 depicts the connection of a predictive failure monitoring system  160  (PFMS  160 ), in accordance with one embodiment of the present invention, between a mass flow controller  100  and a system controller  164 . Normally, signals are communicated to and from the system controller  164  by way of a system card edge socket  162 , which is normally connected to the MFC card edge connector  108 . Here, the predictive failure monitoring system  160  is connected to the MFC card edge connector  108 , and the system card edge socket  162  is connected to a monitor card edge connector  168 . In normal operation all electrical signals on the MFC card edge connector  108  are passed straight-through to the monitor card edge connector  168 , so that the predictive failure monitoring system  160  may exist transparently in the total system, without requiring any software or controller changes to be made. Moreover, no cabling changes are required, as even power is supplied to the predictive failure monitoring system  160  by the system controller  164 . User-controlled switches  174  (e.g., DIP switches) provide for configuration of the predictive failure monitoring system  160  and a variety of status lights  172  allow an operator to perceive its operational status. An optional LCD display  170  may also be connected to provide user feedback of the operational status of the monitoring system  160 . Moreover, optional pressure inputs  176  afford the capability of monitoring the upstream and/or downstream pressure along with the MFC setpoint voltage and MFC output voltage. Both the upstream and downstream pressures may be monitored, or alternatively the pressure differential across the MFC may be monitored. Optional switch closure output terminals (not shown) provide for a series alarm loop through multiple devices, where an alarm condition within any one device breaks the circuit and triggers a system alarm. These and other capabilities are described in greater detail below. 
     Referring now to FIG. 6, the major components of the predictive failure monitoring system  160  include a CPU  188 , internal storage  186 , user selection interface  190 , status interface  192 , MFC A/D interface  180 , power interface  182 , and pressure A/D interface  184 . The monitoring of the MFC  100  is performed by measuring the MFC setpoint voltage and internal valve voltage at a point in time, and comparing them against the respective values of these two parameters when performance was known to be good. The internal storage  186  serves to store the reference values of the MFC setpoint voltage and the corresponding internal valve voltage when performance of the MFC is known to be good. The CPU  188  performs necessary sequencing and control of the predictive failure monitoring system  160 . The internal storage  186  and CPU  188  may be advantageously implemented, for example, using a flash E 2 PROM microcontroller, such as is available from the Microchip Corporation of Phoenix, Ariz. The user selection interface  190  receives configuration inputs from a user (by way of, for example, switches or a computer interface), as described below. The status interface  192  provides (by way of, for example, display lights, a computer interface, or an attached LCD display  170 ) status information about the attached MFC. The MFC A/D interface  180  connects to various signals passing between the system controller  164  and the MFC  100 , digitizes certain ones, particularly the MFC setpoint voltage and the valve voltage, and provides the digitized values to the CPU  188 . Power from the system controller  164  is received into the power interface  182  and provided to other portions of the predictive failure monitoring system  160 . Lastly, signals from optional external pressure transducers (indicated as gas pressure transducers  194 ) are received by pressure A/D interface  184 , digitized, and provided to the CPU  188 . 
     The monitoring of the MFC  100  is performed by measuring the MFC setpoint voltage and internal valve voltage at a point in time, and comparing them against the respective values of these two parameters when performance was known to be good. If the measured valve voltage for a given MFC setpoint voltage no longer holds within predetermined limits of the reference value of the valve voltage, various actions can occur, including generating an alarm. Optionally, the reference relationship may include both upstream and downstream gas pressure measurements. The reference relationship may be table-based, where the table may be loaded into internal storage  186  in any of several ways, including an automatic characterization (“learn mode”) of the attached MFC, or an automatic statistical storing of actual MFC setpoint voltages used by the system controller  164  and the resulting valve voltages generated by the MFC  100 . Moreover, the reference relationship may be equation based, all as described more fully below. 
     In all these operating modes, the basic operation of the predictive failure monitoring system  160  may be described as follows. Measurements are performed at a usually regular interval such as, for example, every 16 ms. For each measurement of the MFC setpoint voltage, the nominal expected value of the valve voltage (i.e., the “model” value) is determined either by indexing into a table (stored, typically, in internal storage  186 ) or by computation using a characterization equation for the particular MFC. The measured valve voltage is then compared against a user-settable tolerance window centered around the nominal expected valve voltage just determined. If the measured valve voltage falls outside the tolerance window, then an “OUT” counter is incremented. If the next measurement is determined to also fall outside the tolerance window, then the OUT counter is incremented again. The OUT counter will continue to increment if each consecutive sample (or measurement) falls outside the tolerance window. However, if a sample finds the measured value of valve voltage within the tolerance window, then the OUT counter is reset and an IN counter is incremented. Subsequent measurements within the tolerance window increment the IN counter. In this way, momentary measurements which fall outside of the tolerance window may be ignored, while only continued out-of-tolerance conditions advance the status of the predictive failure monitoring system  160  to more urgent status levels. 
     In the exemplary embodiment of the predictive failure monitoring system  160  which is shown, four user-selectable tolerance settings are provided. For each setting, a CONTROL limit and an ALARM limit are specified. A warning is typically generated by the predictive failure monitoring system  160  if operation of the MFC  100  falls outside of a CONTROL tolerance window, while an alarm condition is typically generated if operation of the MFC  100  falls outside of an ALARM tolerance window. The four exemplary tolerance settings are set forth in Table 1 and are selectable by the user, by way of switches, a computer interface, or other suitable manner. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Control 
                 Alarm 
               
               
                   
                   
               
             
             
               
                   
                 ±2% 
                  ±5% 
               
               
                   
                 ±5% 
                 ±10% 
               
               
                   
                 ±5% 
                 ±15% 
               
               
                   
                 ±10% 
                 ±20% 
               
               
                   
                   
               
             
          
         
       
     
     A visual indication of the operation of the attached MFC is provided by a group of operational status lights  172  which include, for example, a green, a yellow, and a red light. The green light is illuminated when measured operation of the MFC  100  is within the CONTROL tolerance window (i.e., “within CONTROL limits”). The yellow light is illuminated when measured operation of the MFC  100  is outside the CONTROL tolerance window but is within the ALARM tolerance window (i.e., “within ALARM limits”). Lastly, the red light is illuminated when measured operation of the MFC  100  is outside the ALARM tolerance window. These status lights  172  are all provided within the status interface  192 . Moreover, a dry contact switch is also provided by status interface  192  for integration into an alarm loop. The switch is closed when alarm limits are exceeded for the requisite number of consecutive samples. 
     A predetermined number of consecutive samples must all be outside the CONTROL limits for the status of the monitoring system  160  to transition from “green” to “yellow” status. This number of consecutive samples is user-selectable. Likewise, the same user-selectable number of consecutive samples must be outside the ALARM limits for the status of the monitoring system  160  to transition to “red” status. If, at any time, a given number of consecutive samples are within a particular tolerance window, the status advances to correspond to that tolerance window. For example, if the predictive failure monitoring system  160  currently indicates an alarm condition (e.g., a “red” status), a user-selectable number of samples falling within the ALARM limits causes the status to advance from “red” to “yellow.” Similarly, the same user-selectable number of samples falling within the CONTROL limits causes the status to advance to “green.” Four different user-selectable delay/sample sizes are provided in the exemplary embodiment, as summarized in Table 2 below. 
     
       
         
               
               
             
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 OUT Samples 
                 IN Samples 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 2 
                  1 = 16 ms 
               
               
                 4 
                  2 = 32 ms 
               
               
                 16 
                  8 = .125 s 
               
               
                 32 
                 16 = .25 s 
               
               
                   
               
             
          
         
       
     
     A particular advantage of the exemplary monitoring system  160  is its transparent monitoring of an attached MFC. No changes in the system controller are required, for the predictive failure monitoring system  160  resides between the system controller  164  and the MFC  100 . All signals on the MFC card edge connector  108  (with one exception to be described below) are passed through without change to the monitor card edge connector  168 . This allows the predictive failure monitoring system  160  to be directly plugged into an existing system. Some MFC systems can use the existing valve test point to measure the valve voltage signal, while others will require the valve voltage signal to be brought to and connected to the MFC card edge connector  108 . 
     Characterization of an attached MFC may be performed during MFC calibration and a resulting equation embedded into the operating code of the predictive failure monitoring system  160 . Rather than equation based, the predictive failure monitoring system  160  may also be table-based. Characterization can be performed in-situ (i.e., the “learn mode”) by collecting a spread of actual measured data into an internal table for later use in monitoring. Alternatively, characterization can be performed ad hoc by collecting only MFC setpoint voltage data used by the system controller  164  and measuring the resulting valve voltage (i.e., the “watch mode”). In either mode, the table of data may be dumped under user control to a host computer or other suitable controller. Each of these operating modes is described in greater detail below. 
     Referring now to FIG. 7, the major operating blocks of the predictive failure monitoring system  160  are described in flowchart form. After power-up initialization at block  200 , the tolerance settings and the operating mode settings are read. If operation is to be equation based, flow passes from block  202  to block  204  and the predictive failure monitoring system  160  operates using predefined equations (one of two “run” modes). If operation is to be table-based, flow passes from block  206  to block  208  and the predictive failure monitoring system  160  operates using established table data previously loaded into the internal storage  186  (the second of the two “run” modes). However, if the table is to be built sequentially, control passes from block  210  to block  212  and the MFC is cycled to build the table. Control passes through block  214  to block  216  if the table is to be built on the fly (i.e., the “ad hoc” or “watch” mode) using actual settings from the system controller  164 . Alternatively, if none of these modes is selected by the combination of user switches, then control passes to block  218  and the table is dumped through an interface (e.g., an RS-232 interface, optionally implementing the Semiconductor Equipment Communication Standard (SECS) protocol) to an attached computer or other suitable controller. 
     The flow details of operation using predefined equations, block  204 , is shown in FIG.  8 . The MFC setpoint voltage and the actual measured valve voltage “V(mfc)” is obtained at step  220 . Optional pressure inputs are obtained at step  222 . The nominal valve voltage “V(calc)” is then calculated at step  224  using the MFC setpoint voltage and the stored equation (and possibly using the optional pressure inputs as well). The various parameters may be displayed on the optional LCD display at step  225 . Steps  226  and  228  serve to determine whether the measured valve voltage “V(mfc)” falls within the user-selectable CONTROL limits (e.g., within the tolerance window of the nominal calculated valve voltage “V(calc)” plus and minus the “Control %”). If so, control passes to step  230  where the IN counter is incremented and the OUT counter is cleared. If the IN counter is greater than or equal to the threshold value, control passes from step  232  to step  234  where the green light is illuminated, the IN counter is cleared, and control is returned to step  220  to begin another sample. However, if the measured valve voltage falls outside the user-selectable CONTROL limits, control passes to step  236  where the OUT counter is incremented and the IN counter is cleared. Steps  238  and  240  then serve to determine whether the measured valve voltage falls within the user-selectable ALARM limits (the “Alarm %”). If so, control passes to step  248  to test the OUT counter. If the OUT counter is greater than or equal to the threshold value, control passes to step  250  where the yellow light is illuminated, the OUT counter is cleared, and control is returned to step  220  to begin another sample. If the OUT counter is less than the threshold value, control is immediately returned to step  220  to begin another sample. However, if (at steps  238 ,  240 ) the measured valve voltage falls outside the user-selectable ALARM limits, control passes to step  242  to test the OUT counter. If the OUT counter is less than the threshold value, control is immediately returned to step  220  to begin another sample. Alternatively, if the OUT counter is greater than or equal to the threshold value, control passes to step  244  where the red light is illuminated, the relay is turned on, the OUT counter is cleared, and control is returned to step  220  to begin another sample. An optional step  246  requires manual clearing of the alarm condition by a user before control is returned to step  220 , to ensure that even a brief alarm condition is noted by user intervention. 
     The flow details of operation using table-based data, block  208 , is shown in FIG.  9 . The steps  260 - 290  are generally analogous to respective steps  220 - 250  in FIG.  8 . The MFC setpoint voltage and the actual measured valve voltage “V(mfc)” is obtained at step  260 . Optional pressure inputs are obtained at step  262 . The nominal valve voltage “V(tbl)” is then determined from the stored table at step  264  using the MFC setpoint voltage to index into the stored table and then adjusted by the optional pressure inputs. The various parameters may be displayed on the optional LCD display at step  265 . Steps  266  and  268  serve to determine whether the measured valve voltage “V(mfc)” falls within the user-selectable CONTROL limits (e.g., within the tolerance window of the nominal table-determined valve voltage “V(tbl)” plus and minus the “Control %”). If so, control passes to step  270  where the IN counter is incremented and the OUT counter is cleared. If the IN counter is greater than or equal to the threshold value, control passes from step  272  to step  274  where the green light is illuminated, the IN counter is cleared, and control is returned to step  260  to begin another sample. However, if the measured valve voltage falls outside the user-selectable CONTROL limits, control passes to step  276  where the OUT counter is incremented and the IN counter is cleared. Steps  278  and  280  then serve to determine whether the measured valve voltage falls within the user-selectable ALARM limits (the “Alarm %”). If so, control passes to step  288  to test the OUT counter. If the OUT counter is greater than or equal to the threshold value, control passes to step  290  where the yellow light is illuminated, the OUT counter is cleared, and control is returned to step  260  to begin another sample. If the OUT counter is less than the threshold value, control is immediately returned to step  260  to begin another sample. However, if (at steps  278 ,  280 ) the measured valve voltage falls outside the user-selectable ALARM limits, control passes to step  282  to test the OUT counter. If the OUT counter is less than the threshold value, control is immediately returned to step  260  to begin another sample. Alternatively, if the OUT counter is greater than or equal to the threshold value, control passes to step  284  where the red light is illuminated, the relay is turned on, the OUT counter is cleared, and control is returned to step  260  to begin another sample. Optional step  286  requires manual clearing of the alarm condition by a user before control is returned to step  260 , as before. 
     The flow details of operation to create or build a table, block  212 , is shown in FIG.  10 . In this mode the monitoring system  160 , rather than the system controller  164 , temporarily drives the MFC setpoint voltage input across a range of values and reads the corresponding valve voltage generated by the MFC  100 . Consequently, at step  300  the MFC setpoint voltage jumper is changed to disconnect the MFC setpoint voltage signal conveyed to MFC card edge connector  108  from the monitor card edge connector  168 , and instead connects it to be driven by the monitoring system  160 . This may be accomplished, for example, by a jumper or a switch. At step  302  the data table is then cleared, both red and green lights are illuminated to indicate to a user that the monitoring system  160  is currently building a table, and the collection process is initialized. The table may optionally be left unerased at step  302  to allow a partial table to be built. A loop is created by steps  304 ,  306 ,  308 ,  310 ,  312 , and  314  to incrementally drive a range of MFC setpoint voltages to the attached MFC, and to measure and store the resulting valve voltage generated by the MFC corresponding to each MFC setpoint voltage. After incrementing the MFC setpoint voltage at step  310 , the value is checked against a MAX value at step  312 . If not yet finished, the green light is toggled at step  314  and control returned to step  304  to drive the incremented MFC setpoint voltage to the attached MFC and to measure and store the corresponding valve voltage. When the range of MFC setpoint voltage is exceeded at step  312 , control passes to step  316  to optionally measure and store the reference upstream and downstream pressures associated with the measured data table. Then, control passes to step  318  where the yellow light is continuously toggled to indicate the table build is complete, and to alert a user to reset the monitoring system  160 . 
     The flow details of operation to collect data ad hoc (the “watch mode”), block  216 , is shown in FIG.  11 . At step  320  the data table is cleared. At step  322  an MFC setpoint voltage is obtained (received from the system controller  164 ), and a corresponding valve voltage generated in response is measured and obtained at step  324 . The obtained MFC setpoint voltage is then used, at step  326 , to calculate a storage location and thereby index into the data table. If data is found in the calculated storage location corresponding to the MFC setpoint voltage, the data is retrieved at step  330 , averaged with the newly measured valve voltage, and stored back into the respective location at step  332 . Alternatively, rather than computing and storing such a “running” average, a weighted average of valve voltages may also be computed by allocating additional storage locations within the internal storage  186  to store the sample size for each measurement along with a weighted average value. Since only valve voltage data is stored which corresponds to actual MFC setpoint voltages provided by system controller  164 , if during a subsequent “run” mode, an MFC setpoint voltage is provided to the predictive failure monitoring system  160 , no data will be found in the table, and the valve voltage will be considered outside the ALARM limits. 
     The flow details of operation to dump the data table, block  218 , is shown in FIG.  12 . At step  340  a location pointer is cleared. A loop is created by steps  342 ,  344 ,  346 , and  348  to obtain the MFC valve voltage “V(mfc)” from the data table corresponding to the location pointer, to send the valve voltage data out an RS-232 port, and to increment the location pointer. If any locations remain, control branches from step  348  back to step  342 . If no locations remain, control passes to step  350  where the yellow light is continuously toggled to indicate the table dump is complete, and to alert a user to reset the monitoring system  160 . Using such an operational flow, a table which has been previously collected or established by any of these methods may be dumped (i.e., downloaded to a system controller). 
     Referring now to FIG. 13, an optional LCD display  170  may be used to display various operational parameters of an attached MFC. The LCD display which is available as part number BPK-216N from the Scott Edwards Electronics Company may be used. The LCD display  170  may be connected to the predictive failure monitoring system  160  through a terminal connector on the side of the board. 
     The LCD display  170  shows four parameters relevant to the operation of the MFC and PFMS, as shown in FIG.  13 . The “V=xx.yy” displays the absolute value of the current measured valve voltage as generated by the attached MFC (or by an attached diagnostic test box). While the valve voltage is usually a negative number, the negative sign is dropped for display purposes. The “M=xx.yy” displays the absolute value of the model (i.e., nominal expected) valve voltage. The source of the model valve voltage depends upon the operating mode. For equation-based operation, the model valve voltage is calculated from the equation using the MFC setpoint voltage, and optionally adjusted by including compensation for pressure. For table-based operation, the model valve voltage is looked up from the stored data using the MFC setpoint voltage as an index into the data table. The “S=x.yyy” displays the current MFC setpoint voltage as monitored by the monitoring system  160 , with 0 corresponding to no gas flow, and 5,000 volts corresponding to full scale flow of the MFC. The “%=xx.yy” displays the error percentage of the current valve voltage compared to the model valve voltage, according to the equation: 
     
       
         %=100*(valve−model)/model.  
       
     
     In the table build mode (block  212 ) and the “ad hoc” mode (block  216 ), the LCD display  170  shows the instantaneous MFC setpoint voltage and the corresponding measured valve voltage. 
     An electrical schematic diagram of the exemplary predictive failure monitoring system  160  is shown in FIG.  14 . The LTC1257 device (shown as element  402 ) is a digital-to-analog converter (D/A) used for controlling the MFC setpoint during the in-situ characterization. Jumper JP 1  (shown as element  404 ) intercepts this signal which normally is passed directly from the system controller (received on terminal BA) to the MFC on terminal JA. When jumper JP 1  is in the alternate position, the D/A  402  provides the setpoint to the MFC so that the table can be built sequentially for “in situ” characterization of the MFC. 
     Two LTC1298 devices (shown as elements  406  and  408 ) are analog-to-digital (A/D) converters for the Setpoint, Valve Voltage, and Pressure inputs. A/D  406  digitizes the optional pressure inputs received on terminals  410 ,  412 . A/D  408  digitizes the valve voltage received from the MFC on one of terminals  414 . Since both the A/D and D/A functions need to be done with high accuracy (e.g., in the 1.2 millivolt step size), the LTC1027 device (indicated as element  416 ) is a precision reference voltage generator which supplies a high accuracy 5.00 volt reference voltage for both the A/D devices  406 ,  408  and the D/A device  402 . The 7805 voltage regulator (shown as element  418 ) provides a 5.0 volt supply to the various circuits in the PFMS  160 . 
     Two different signal conditioning circuits  420 ,  422  are provided to convert the valve voltage signal into a 0 to +5 volt signal for the A/D converter  408 . The most common valve voltage signal is a 0 to −15 volt signal. The signal conditioning circuit  420  is designed to convert such a 0 to −15 volt signal into 0 to +5 volt signal for the A/D converter  408 . Since other valve voltage signal ranges are also encountered in commercial products, the signal conditioning circuit  422  is designed to accept either a 0 to +5 volt input signal or a 0 to −5 volt input signal, and output a 0 to about +5 volt signal. These circuits  420 ,  422  may be easily modified to provide any other conversion needed since the bipolar to positive conversion is component heavy, there is easily some flexibility in the printed circuit board such that a number of different circuit configurations may be provided. In other words, the signal conditioning circuit  422  can be configured, by changing components and using jumpers, for converting basically any valve voltage range into a 0 to +5 volt signal without having to modify the basic circuit board layout and design. This provides a cost effective and flexible printed wiring board. Jumper JP 2  (shown as element  436 ) is configured to select which terminal position the valve voltage is received on, and to convey the valve voltage to a selected one of the two signal conditioning circuits  420 ,  422 . Jumper JP 4  (shown as element  424 ) is configured to select the output of one of the two signal conditioning circuits  420 ,  422 , and convey it to the A/D converter  408 . Jumper JP 3  (shown as element  430 ) is used to select the source of the setpoint signal to the PFMS. The setpoint signal source may be configured to be the input signal from the system controller  164  to the MFC, or the setpoint signal source may be configured to be the feedback signal from the MFC itself. 
     Switches S 0 -S 3  and their respective resistors together form a first resistor/switch network  426  which is used to select the operating tolerance as illustrated in Table 1 and Table 2 above. Switches S 4 -S 7  and their respective resistors together form a second resistor/switch network  428  which is used to select the operating mode, as illustrated in FIG.  7 . Alarm circuit  432  provides the relay output which provides for configuring a series alarm loop. Terminals  434  provide the connection path the optional LCD display  170 . 
     FIGS. 15A and 15B each show the placement of the various components on the printed wiring board  126  of the predictive failure monitoring system  160 , including a reset pushbutton  500  and two possible locations for the status lights  172 . The side connector  502  is also shown for attaching the optional LCD display  170  and pressure inputs  176 . 
     While the invention has been described with respect to the embodiments set forth above, the invention is not necessarily limited to these embodiments. For example, any of a wide variety of data collection interfaces may be employed in addition to or instead of the LCD described, such as ethernet, sensorbus, or SECS-II (which uses RS-232 cabling, but an industry agreed-upon signaling protocol). Moreover, control of the predictive failure monitoring system  160  may be initiated not by local switches manipulated by a user, but by way of commands received from a suitable computer or control interface. While the accuracy of the MFC monitoring is more accurate by using the optional gas pressure transducers, their use is not mandatory. Variations in design or configuration details, and in the operational flowcharts are contemplated by one skilled in the art using the teachings of this disclosure. 
     In addition, the teachings of this invention are limited to application only to mass flow controllers, but may be advantageously utilized in other feedback controller situations. Accordingly, other embodiments, variations, and improvements not described herein are not necessarily excluded from the scope of the invention, which is defined by the following claims.