Patent Application
20130037112
The present invention relates generally to mass flow controllers. In particular, but not by way of limitation, the present invention relates to methods and systems for controlling a mass flow controller.
A typical mass flow controller (MFC) is a closed-loop device that sets, measures, and controls the flow of a gas in industrial processes such as thermal and dry etching among other processes. An important part of a MFC is a sensor that measures the mass flow rate of the gas flowing through the device. Typically, a closed-loop control system of the MFC compares an output signal from the sensor with a predetermined set point and adjusts a control valve to maintain the mass flow rate of the gas at the predetermined set point.
The closed-loop system performs well when valve is relatively close to required position and its movement changes the flow, so that the algorithm immediately sees flow response and adjusts valve position accordingly. When a MFC is at zero set point (zero valve position), and then is given non-zero set point, it takes a long time for valve to move from zero position to a position where noticeable flow appears and closed-loop algorithm starts working properly. This results in long response delay and poor performance of the MFC. And to date, attempts to improve a response time of the MFC have been unsuccessful or are otherwise unsatisfactory.
Illustrative embodiments of the present invention that are shown in the drawings are summarized below. These and other embodiments are more fully described in the Detailed Description section. It is to be understood, however, that there is no intention to limit the invention to the forms described in this Summary of the Invention or in the Detailed Description. One skilled in the art can recognize that there are numerous modifications, equivalents, and alternative constructions that fall within the spirit and scope of the invention as expressed in the claims.
An aspect of the invention may be characterized as a mass flow controller comprising a valve that is adjustable between a closed position and an open position to control a flow rate of a fluid responsive to a control signal, a thermal mass flow sensor that provides an indication of the flow rate of the fluid, calibration data including data that relates the control signal to the flow rate of the fluid at a plurality of fluid flow rates, and a control system that provides, based upon the calibration data and run time data, an adjustable non-zero starting control signal to the valve, when the valve is closed, to more quickly respond to the set point signal.
Another aspect of the invention may be characterized as a method for operating a mass flow controller. The method may include receiving, when the valve is closed, a set point signal corresponding to a desired flow rate and calibration data stored on the mass flow controller to obtain a value of a calibrated non-zero starting control signal and to obtain a value of a calibrated control signal at a particular flow rate. In addition, an adjustable non-zero starting control signal is applied at the value to a valve of the mass flow controller and during operation, and before the set point signal decreases, a measured value of the control signal at the particular flow rate is obtained. The measured value of the control signal is compared with a level of a calibrated control signal at the particular flow rate that is stored on the mass flow controller, and based upon the comparison, the value of the adjustable non-zero starting control signal is adjusted to an adjusted value so that a next time the mass flow controller receives, when the valve is closed, another set point signal, the adjusted value is used.
These and other embodiments are described in further detail herein.
Various objects and advantages and a more complete understanding of the present invention are apparent and more readily appreciated by reference to the following Detailed Description and to the appended claims when taken in conjunction with the accompanying Drawings, wherein:
Referring now to the drawings, where like or similar elements are designated with identical reference numerals throughout the several views where appropriate, and referring in particular to
As depicted, in the present embodiment a base 105 of MFC 100 includes bypass 110 through which a gas flows. Bypass 110 directs a constant proportion of gas through main path 115 and sensor tube 120. As a consequence, the flow rate of the fluid (e.g., gas or liquid) through the sensor tube 120 is indicative of the flow rate of the fluid flowing through the main path of the MFC 100.
In this embodiment, the sensor tube 120 is a small bore tube that is part of a thermal mass flow sensor 123 of the MFC 100. And as shown, sensing elements 125 and 130 are coupled to (e.g., wound around) the outside of sensor tube 120. In one illustrative embodiment, sensing elements 125 and 130 are resistance-thermometer elements (e.g., coils of conductive wire), but other types of sensors (e.g., resistance temperature detectors (RTD and thermocouples) may also be utilized. Moreover, other embodiments may certainly utilize different numbers of sensors and different architectures for processing the signals from the sensors without departing from the scope of the present invention.
As depicted, sensing elements 125 and 130 are electrically connected to a sensing-element circuit 135. In general, the sensing-element circuit 135 is configured (responsive to signals 146, 148 from the sensing elements 125, 130) to provide an output signal 150, which is indicative of the flow rate through the sensor tube 120, and hence, indicative of the flow rate through the main path 115 of the MFC 100.
As shown in
As one of ordinary skill in the art will readily recognize, the processing portion 160 may also adjust the output signal 150 (e.g., by adjusting the signal 150 with predetermined calibration coefficients) based upon physical characteristics of the MFC 100 and/or characteristics of the fluid (e.g., gas) flowing through the MFC 100.
The control component 170 in this embodiment is generally configured to generate a control signal 180 to control a position of the control valve 140 based upon the output signal 150. The control valve 140 may be realized by a piezo valve or solenoid valve, and the control signal 180 may be a voltage (in the case of a piezo valve) or current (in the case of a solenoid valve). And as one of ordinary skill in the art will appreciate, the MFC 100 may include pressure and temperature sensors that provide pressure (P) and temperature (T) inputs to the control component 170. And as shown, the control component 170 in this embodiment includes an adaptive valve start component 182 that is coupled to calibration data 184, a user input, and adjustment data 185. Collectively, the processing 160, control component 170 (including the adaptive valve start component 182), and the control signal 180 generated by the control component 170 are part of a control system that provides, based upon the calibration data and run time data, an adjustable non-zero starting control signal to the valve 140 when the valve 140 is closed to more quickly respond to the set point signal 186. The control component 170 may be realized by software, nonvolatile memory, hardware and/or firmware or combinations thereof, and it may include non-transitory processor readable instructions that effectuate the methods described further herein.
In general, the adaptive valve start component 182 operates to provide an adjustable non-zero starting control signal 180 to the control valve 140, based upon the calibration data 184 and runtime data of the MFC 100, when the control valve 140 is closed, to more quickly respond to a set point signal 186. In addition, the user input to the adaptive valve start component 182 enables a user to alter the adjustable non-zero starting control signal 180 to adjust a response of the MFC 100 as discussed further with reference to
In prior implementations, the closed-loop control loop of mass flow controllers performed relatively well when the valve is relatively close to a required position and its movement changes the flow, so that the control loop sees flow response and immediately adjusts the valve position accordingly. But in these prior systems, when the MFC was set to a zero position (zero valve position), and the MFC was given a non-zero set point, it would take a long time for the valve to move from a zero position to a position where a noticeable flow would appear and the closed-loop control loop would start working properly. As a consequence, there was a long response delay and generally poor MFC performance.
Thus, to remove the response delays and poor performance, the adaptive valve start component 182 improves the performance of the MFC 100 by immediately moving the control signal 180 from a zero value (e.g., zero current or voltage) to an adjustable non-zero starting control signal value while the control valve 140 is closed. Referring briefly to
In many variations of the embodiment depicted in
Referring next to
As shown, in the process depicted in
Once the initial non-zero starting control signal value is stored, additional increasing values of the control signal 180 are applied (Block 310), and for each of the control signal values, the measured flow rate is stored (in connection with the corresponding control signal value) to create data pairs for the particular pressure (Block 306). For example, once flow is first detected to obtain a starting control signal value, the control signal may be increased in a stepwise manner (e.g., by a step size of 1% of the maximum control signal level) until the maximum flow or maximum control signal level is reached.
It is noted that an actuator associated with the MFC 100 experiences hysteresis (e.g., piezoelectric or solenoid hysteresis) so that control signal values on a curve of ascending flow values differ from the control signal values for the same flow values on a curve of descending flow values. Referring to
Referring again to
Referring for example to
Referring next to
As shown, the calibration data 184 is accessed to obtain a value of a calibrated non-zero starting control signal and a value of a calibrated control signal at a particular flow rate, and these values are used later to adjust the adjustable non-zero starting control signal (Block 604). The control signal 180 is then applied as an adjustable non-zero starting control signal at an initial value to the control valve 140 (Block 606). As a consequence, the closed-loop control system of the MFC 100 is engaged substantially sooner (when the flow is about to start or has just started) as opposed to prior approaches where the starting control signal value is zero and the control loop is not engaged until after a delay during which the control signal slowly reaches a level (using the control loop) where the flow begins.
When the MFC 100 is first deployed for use (e.g., when a user receives the MFC 100 from a supplier), the calibrated non-zero starting control signal may be used as the initial value of the adjustable non-zero starting control signal, but once the MFC 100 is in use, the adjustable non-zero starting control signal is based upon the calibration data and run time data.
For example, in embodiments where the calibration data 184 includes calibration data for a plurality of pressures, the control signal 180 is applied at Block 606 as an adjustable non-zero starting control signal at a value that is obtained by adding difference data (stored in the adjustment data 185) to the calibrated non-zero starting control signal. The difference data in these embodiments is based upon differences between the calibration data 184 and run time measurements that were previously obtained during one or more previous process runs. Additional information detailing an exemplary approach for generating the difference data is provided below with reference to Blocks 610 and 612 below.
And in the embodiments where the calibration data 184 includes calibration data for only a single pressure, the adjustment data 185 includes the value of the adjustable non-zero starting control signal, and the control signal 180 is applied at Block 606 as an adjustable non-zero starting control signal at the value obtained from the adjustment data 185. As discussed below with reference to Blocks 610 and 612, the stored value of the adjustable non-zero starting control signal may be adjusted during each run and updated in the adjustment data 185.
Regardless of whether the calibration data 184 is based upon a single pressure or multiple pressures, the value of the calibrated control signal (at a particular flow rate) that is obtained in Block 604 is utilized, as discussed further below, to adjust the adjustable non-zero starting control signal during a subsequent run. Although two pieces of data are obtained in Block 604, it should be recognized that these two pieces of data need not be obtained co-currently.
In the implementations where the calibration data 184 includes calibration data for each of a plurality of pressure levels (e.g., as depicted in
But having calibration data for a plurality of pressures is not required in connection with the method depicted in
Although applying an adjustable non-zero starting control signal to the MFC 100 when the control valve 140 is closed will generally improve a response of the MFC 100, it is contemplated that users of the MFC 100 will desire a particular transient response depending upon the particular processing application in which the MFC 100 is used. As a consequence, in many embodiments the adaptive valve start component 182 enables a user to define (by way of the user input) a desired transient response of the MFC 100 by adding or subtracting an offset from the adjustable non-zero starting control signal.
Referring to
Similarly, if the non-zero starting control signal provides the response shown in
Although the adjustable non-zero starting control signal generally improves response, and may be configured to arrive at a desired transient response, environmental (e.g., temperature) and other factors (e.g., age of the MFC 100) affect the relationship between the transient response and the starting control signal. In other words, if a desired transient response is achieved (e.g., by adjustment with an offset that is applied to the starting control signal), temperature and age will cause the MFC 100 to have a different response with the same starting control signal.
Referring to
Most of the time, a temperature and/or aging-related change of valve-flow characteristics is practically a “parallel shift,” that may characterized by a curve that shifts left or right along a “control signal” axis while its shape stays substantially the same. Referring to
As a consequence, as part of the adjustment to the adjustable non-zero starting control signal, during operation, before the set point signal 186 decreases, a measured value of the control signal is obtained at the particular flow rate (Block 608). The particular flow rate at which the measured flow rate is obtained is the same particular flow rate (discussed with reference to block 604) that was used in connection with obtaining the value of the calibrated control signal from the calibration data 184 in block 604 above. And the measured value is obtained before the set point 186 decreases so that the measured value is taken from an ascending control-signal-versus-flow curve (just as the calibrated control signal at the particular flow rate was obtained during calibration).
Referring to
As shown in
In many embodiments, the value of the adjustable non-zero starting control signal is adjusted based upon the following algorithm: ASCS=CSCS+MVCS−CVCS where ASCS is the adjustable non-zero starting control signal that is adjusted to maintain a desired response; CSCS is the calibrated starting control signal, which is the value of the starting control signal taken from the calibration data; MVCS is the measured value of the control signal that is measured at a particular flow level; and CVCS is the calibrated value of the control signal, which is the value of the calibrated control signal at the particular flow value.
With reference to
In embodiments where the calibration data 184 includes calibration data for a plurality of pressures, the difference between the measured value of the control signal (MVCS) and the calibrated value of the control signal (CVCS) is stored in the adjustment data 185 so that during a subsequent run, the stored difference is added to the value of the calibrated non-zero starting control signal that is stored in the calibration data 184 (for the current pressure) to obtain the adjustable non-zero starting control signal (ASCS). And the method described above with reference to Blocks 608 to 612 is carried out again to adjust the difference data as needed for yet other subsequent process runs.
And in the embodiments where the calibration data 184 includes calibration data for only a single pressure, the adjustment data 185 includes the value of the adjustable non-zero starting control signal (ASCS), which is accessed during a subsequent process run (the same way the initial value of a calibrated non-zero starting control signal is accessed as described with reference to Block 604), and applied to the control valve 140 as the adjustable non-zero starting control signal as discussed above with reference to Block 606. And the method described above with reference to Blocks 608 to 612 is carried out again to adjust the adjustable non-zero starting control signal as needed.
In variations of the method depicted in
Although the method described with reference to
For example, when a new adjustable non-zero starting control signal value (or difference data) is stored in the adjustment data 185, a temperature value from a temperature sensor in the MFC 100 may also be stored so that temperature information is stored in connection with the starting control signal value or difference data. The stored temperature data (in connection with the control signal or difference data) can be used to predict the optimal adjustable non-zero starting control signal value for subsequent process runs instantaneously if the temperature of the gas has changed significantly between the process runs.
In conclusion, the present invention provides, among other things, a method and apparatus for providing a desired response in a mass flow controller by providing an adjustable non-zero starting control signal to a control valve of the MFC, and maintaining the desired response by adjusting the starting control signal when factors such as temperature and age affect the desired response. Those skilled in the art can readily recognize that numerous variations and substitutions may be made in the invention, its use, and its configuration to achieve substantially the same results as achieved by the embodiments described herein. Accordingly, there is no intention to limit the invention to the disclosed illustrative forms. Many variations, modifications, and alternative constructions fall within the scope and spirit of the disclosed invention as expressed in the claims.
