Abstract:
An apparatus of mass flow controlling for use in an integrated gas delivery system, comprising an input terminal, a sensor unit, an electromagnetic valve, and a control unit. The control unit comprises an A/D converter, a microprocessor, and a valve control circuit. The A/D converter converts a flow rate setting signal inputted by the input terminal into a first digital signal, and converts a flow rate detection signal outputted by the sensor unit into a second digital signal. The microprocessor further comprises a control module and a calculation module. The valve control circuit opens the electromagnetic valve according to the first control signal only, and further regulates an openness of the electromagnetic valve according to the first control signal and the second control signal. It is concluded that response time of the mass flow control apparatus of the present invention is shorten, and control quality is improved.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority benefit of International Patent Application Serial No. PCT/CN2014/080690, filed Jun. 25, 2014, which is related to and claims the priority benefit of China patent application serial No. 201410260751.7, filed Jun. 12, 2014. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to the field of semiconductor manufacturing technology, and particularly to a method and an apparatus of mass flow controlling for use in an integrated gas delivery system. 
       BACKGROUND OF THE INVENTION 
       [0003]    Measuring and controlling of a flow are crucial contents for an integrated gas delivery system. One commonly used device is a mass flow controller (MFC), which controls the mass of the introduced gas or liquid tightly. Referring to the  FIG. 1 , a block diagram of a prior art MFC is shown. The prior art MFC comprises a sensor  11 , an electromagnetic valve  12 , a sensor driver circuit  14  coupled to the sensor  11  in order to receive a detection signal from that, an electromagnetic valve driver circuit  15  coupled to the electromagnetic valve  12  in order to adjust a flow through that, a microprocessor  13  coupled to the sensor driver circuit  14  and the electromagnetic valve driver circuit  15  respectively, and an A/D converter  16 . Various parameters of the gas or the liquid introduced into a tubular shunt  17 , such as a flow, a flow rate, etc., are sensed by the sensor  11  and converted into an electronic signal to be outputted to the sensor driver circuit  14  for processing. The A/D converter  16  converts an inputted setting signal into a first digital signal, and converts the processed signal outputted by the sensor driver circuit  14  into a second digital signal. The microprocessor  13  is coupled to the A/D converter  16  for receiving the first digital signal and the second digital signal to generate a flow control signal. Then, the electromagnetic valve driver circuit  15  converts the flow control signal outputted by the microprocessor  13  into an analog signal to control the electromagnetic valve  12 , so as to control the flow and the flow rate of the gas or the liquid. Referring to the  FIG. 2 , which is a schematic diagram of a closed-loop circuit according to the prior art MFC. In general, the mechanism of the prior art MFC is a control system having a closed-loop circuit. A difference in a detection signal outputted by a sensor  21  and a setting signal is calculated by a PID module  22  and converted into a control voltage to control an openness of an electromagnetic valve  23 , so as to control a flow of a fluid  24  tightly. Referring to the  FIG. 3 , a schematic diagram of an electromagnetic valve according to the prior art MFC is shown. The electromagnetic valve comprises an elastic component  31  positioned above a fluid inlet in order to generate an elastic force downward to close the fluid inlet, and an electromagnetic coil  32  around the elastic component  31  in order to generate an electromagnetic force opposite to the elastic force to open the fluid inlet when a current is provided. A balance between the elastic force and the electromagnetic force is required to adjust the height distancing the fluid inlet, so as to stable the flow rate of the fluid. However, the electromagnetic force is not proportionally related to the height. With the change of the height, nonlinear change of the electromagnetic force is obviously exposed out. 
         [0004]    Although the conventional MFC may be adequate for a flow control at the point of 100% of a full scale, it has significant drawbacks of big overshoot and long response time for a flow control at the point of 2% of a full scale. It is difficult for a conventional MFC to ensure a timely control response for a flow rate at any point in a 2% to 100% of a full scale. Therefore, a new MFC is needed to precisely control a flow rate of the gas at any point in a 100% of a full scale, and also needed to meet the needs for various using environments. 
         [0005]    Accordingly, it is an urgent problem to be solved that a new MFC is required to complete a precisely control for a flow rate in a wide scale. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0006]    To overcome the problems as mentioned above, it is an object of the present invention to provide a method and an apparatus of mass flow controlling having a better control quality and shorter response time. 
         [0007]    To achieve above object, the present invention provides a mass flow control apparatus, comprising an input terminal used to input a flow rate setting signal, a sensor unit coupled to a fluid to sense its flow rate and output a flow rate detection signal, an electromagnetic valve coupled to the fluid for regulating its flow rate, and a control unit. The control unit comprises an A/D converter, a microprocessor and a valve control circuit. The A/D converter converts the flow rate setting signal inputted by the input terminal into a first digital signal, and converts the flow rate detection signal outputted by the sensor unit into a second digital signal. The microprocessor is coupled to the A/D converter for receiving the first digital signal outputted by the A/D converter and the second digital signal outputted by the A/D converter, or receiving the first digital signal inputted directly by the input terminal and the second digital signal outputted by the A/D converter. The microprocessor further includes a control module and a calculation module, wherein, the control module is used to generate and output a first control signal according to the corresponding first digital signal outputted by the A/D converter or inputted directly by the input terminal, and the calculation module is used to generate and output a second control signal by running a calculation for a difference in the first digital signal outputted by the A/D converter or inputted directly by the input terminal and the second digital signal outputted by the A/D converter. The valve control circuit is coupled to the microprocessor for opening the electromagnetic valve according to the first control signal only, or regulating an openness of the electromagnetic valve according to the first control signal and the second control signal. 
         [0008]    Preferably, the valve control circuit comprises a first valve control circuit used to receive the first control signal and generate a first openness control signal, a second valve control circuit used to receive the second control signal and generate a second openness control signal, and a third valve control circuit coupled to the first valve control circuit and the second valve control circuit for receiving the first openness control signal and the second openness control signal to control the electromagnetic valve. When only the first openness control signal is received, the first openness control signal is outputted to the electromagnetic valve to control its opening. When the first openness control signal and the second openness control signal are simultaneously received, the superposition of the first openness control signal and the second openness control signal is outputted to the opened electromagnetic valve to control its openness. 
         [0009]    Preferably, the first valve control circuit and the second valve control circuit are designed by the means of D/A or PWM filtering to proceed a digital-to-analog conversion for the first control signal and the second control signal to generate the first openness control signal and the second openness control signal. 
         [0010]    Preferably, the microprocessor also includes a storage module coupled to the control module for storing a valve model which characterizes a correspondence relation between the first digital signal and the first control signal. The control module outputs the first control signal corresponding to the first digital signal based on the valve model. 
         [0011]    Preferably, the valve model is obtained based on pre-collected data. 
         [0012]    Preferably, the valve model is fitted based on the pre-collected data by a fitting function which is a piecewise function or a continuous function. 
         [0013]    The present invention also provides a second mass flow control apparatus, comprising an input terminal used to input a flow rate setting signal, a sensor unit coupled to a fluid to sense its flow rate and output a flow rate detection signal, an electromagnetic valve coupled to the fluid for regulating its flow rate, and a control unit. The control unit comprises an A/D converter, a microprocessor and a valve control circuit. The A/D converter converts the flow rate setting signal inputted by the input terminal into a first digital signal, and converts the flow rate detection signal outputted by the sensor unit into a second digital signal. The microprocessor is coupled to the A/D converter for receiving the first digital signal outputted by the A/D converter and the second digital signal outputted by the A/D converter, or receiving the first digital signal inputted directly by the input terminal and the second digital signal outputted by the A/D converter. The microprocessor further includes a control module and a calculation module, wherein, the control module is used to generate a first control signal according to the corresponding first digital signal outputted by the A/D converter or inputted directly by the input terminal, and the calculation module is used to generate a second control signal by proceeding a calculation for an adjusted difference in the first digital signal outputted by the A/D converter or inputted directly by the input terminal and the second digital signal outputted by the A/D converter by a first coefficient, which is a ratio of the first digital signal and a difference in the first digital signal and the first control signal. The valve control circuit is coupled to the microprocessor for opening the electromagnetic valve according to the first openness control signal generated basing on the first control signal, or regulating an openness of the electromagnetic valve according to the first openness control signal and the second openness control signal. Wherein, the second openness control signal is generated basing on the second control signal and the first coefficient. 
         [0014]    Preferably, the valve control circuit comprises a first valve control circuit used to receive the first control signal and generate the first openness control signal basing on an initial digital-to-analog conversion proportion, a second valve control circuit used to receive the second control signal and generate the second openness control signal basing on a ratio of the initial digital-to-analog conversion proportion and the first coefficient, and a third valve control circuit coupled to the first valve control circuit and the second valve control circuit for receiving the first openness control signal and the second openness control signal to control the electromagnetic valve. When only the first openness control signal is received, the first openness control signal is outputted to the electromagnetic valve to control its opening. When the first openness control signal and the second openness control signal are simultaneously received, the superposition of the first openness control signal and the second openness control signal is outputted to the opened electromagnetic valve to control its openness. 
         [0015]    Preferably, the first valve control circuit and the second valve control circuit are designed by the means of D/A or PWM filtering to proceed a digital-to-analog conversion for the first control signal and the second control signal to generate the first openness control signal and the second openness control signal. 
         [0016]    Preferably, the microprocessor also includes a storage module coupled to the control module and the calculation module for storing a valve model which characterizes a correspondence relation between the first digital signal and the first control signal. The control module generates the first control signal basing on the valve model. The calculation module calculates out the first coefficient basing on the valve model to generate the second control signal. 
         [0017]    Preferably, the valve model is obtained basing on pre-collected data, or the valve model is fitted to a piecewise function or a continuous function based on the pre-collected data. 
         [0018]    The present invention also provides a third mass flow control apparatus, comprising an input terminal used to input a flow rate setting signal, a sensor unit coupled to a fluid to sense its flow rate and output a flow rate detection signal, an electromagnetic valve coupled to the fluid for regulating its flow rate, and a control unit. The control unit comprises an A/D converter, a microprocessor and a valve control circuit. The A/D converter converts the flow rate setting signal inputted by the input terminal into a first digital signal, and converts the flow rate detection signal outputted by the sensor unit into a second digital signal. The microprocessor is coupled to the A/D converter for receiving the first digital signal outputted by the A/D converter and the second digital signal outputted by the A/D converter, or receiving the first digital signal inputted directly by the input terminal and the second digital signal outputted by the A/D converter. The microprocessor further includes a control module and a calculation module. The control module is used to generate a first control signal according to an adjusted decomposition signal by a second coefficient, wherein, the decomposition signal is generated by the first digital signal, and the second coefficient is a ration of the first digital signal and the decomposition signal. The calculation module is used to generate a second control signal by proceeding a calculation for an adjusted difference in the first digital signal outputted by the A/D converter or inputted directly by the input terminal and the second digital signal outputted by the A/D converter by a first coefficient, which is a ratio of the first digital signal and a difference in the first digital signal and the decomposition signal. The valve control circuit is coupled to the microprocessor for opening the electromagnetic valve according to the first openness control signal generated basing on the first control signal and the second coefficient, or regulating an openness of the electromagnetic valve according to the first openness control signal and the second openness control signal. wherein, the second openness control signal is generated basing on the second control signal and the first coefficient. 
         [0019]    Preferably, the valve control circuit comprises a first valve control circuit used to receive the first control signal and generate the first openness control signal basing on a ratio of an initial digital-to-analog conversion proportion and the second coefficient, a second valve control circuit used to receive the second control signal and generate the second openness control signal basing on a ratio of the initial digital-to-analog conversion proportion and the first coefficient, and a third valve control circuit coupled to the first valve control circuit and the second valve control circuit for receiving the first openness control signal and the second openness control signal and outputting them to the electromagnetic valve. When only the first openness control signal is received, the first openness control signal is outputted to the electromagnetic valve to control its opening. When the first openness control signal and the second openness control signal are simultaneously received, the superposition of the first openness control signal and the second openness control signal is outputted to the opened electromagnetic valve to control its openness. 
         [0020]    Preferably, the microprocessor also includes a storage module coupled to the control module and the calculation module for storing a valve model which characterizes a correspondence relation between the first digital signal and the decomposition signal. The control module calculates out the second coefficient basing on the valve model to generate the first control signal. The calculation module calculates out the first coefficient basing on the valve model to generate the second control signal. 
         [0021]    Preferably, the valve model is obtained based on pre-collected data, or the valve model is fitted to a piecewise function or a continuous function based on the pre-collected data. 
         [0022]    The present invention also provides a method of mass flow controlling applied to a mass flow control apparatus, comprising the steps of: 
         [0023]    S 11 , receiving a flow rate setting signal which is used as or converted into a first digital signal; 
         [0024]    S 12 , generating a corresponding first control signal basing on the first digital signal, and opening an electromagnetic valve basing on the first control signal; 
         [0025]    S 13 , sensing a flow rate of a fluid and outputting a flow rate detection signal; 
         [0026]    S 14 , converting the flow rate detection signal into a second digital signal, and generating and outputting a second control signal by a calculation for a difference in the first digital signal and the second digital signal; 
         [0027]    S 15 , controlling an openness of the electromagnetic valve according to the first control signal and the second control signal. 
         [0028]    The present invention also provides a second method of mass flow controlling applied to a mass flow control apparatus, comprising the steps of: 
         [0029]    S 21 , receiving a flow rate setting signal which is used as or converted into a first digital signal; 
         [0030]    S 22 , generating and outputting a corresponding first control signal basing on the first digital signal; 
         [0031]    S 23 , generating a first openness control signal basing on the first control signal, and opening an electromagnetic valve basing on the first openness control signal; 
         [0032]    S 24 , sensing a flow rate of a fluid and outputting a flow rate detection signal; 
         [0033]    S 25 , converting the flow rate detection signal into a second digital signal, and generating a second control signal by a calculation for an adjusted difference in the first digital signal and the second digital signal by a first coefficient, which is a ratio of the first digital signal and a difference in the first digital signal and the first control signal; 
         [0034]    S 26 , generating a second openness control signal according to the second control signal and the first coefficient, and controlling an openness of the electromagnetic valve according to the first openness control signal and the second openness control signal. 
         [0035]    The present invention also provides a third method of mass flow controlling applied to a mass flow control apparatus, comprising the steps of: 
         [0036]    S 31 , receiving a flow rate setting signal which is used as or converted into a first digital signal; 
         [0037]    S 32 , generating a corresponding decomposition signal basing on the first digital signal; 
         [0038]    S 33 , generating a first control signal by adjusting the decomposition signal using a second coefficient; 
         [0039]    S 34 , generating a first openness control signal basing on the first control signal and the second coefficient, and opening an electromagnetic valve basing on the first openness control signal; 
         [0040]    S 35 , sensing a flow rate of a fluid and outputting a flow rate detection signal; 
         [0041]    S 36 , converting the flow rate detection signal into a second digital signal, and generating a second control signal by a calculation for an adjusted difference in the first digital signal and the second digital signal by a first coefficient, which is a ratio of the first digital signal and a difference in the first digital signal and the decomposition signal; 
         [0042]    S 37 , generating a second openness control signal according to the second control signal and the first coefficient, and controlling an openness of the electromagnetic valve according to the first openness control signal and the second openness control signal. 
         [0043]    The present invention has attained a preferable technology effect by utilizing two control signals to control an electromagnetic valve. The first control signal is firstly generated by a flow rate setting signal to open the electromagnetic valve to a certain openness. Then the second control signal is generated by a PID algorithm to regulate the openness of the opened electromagnetic valve. Comparing to the prior art MFC, in which the openness is opened and regulated only by the PID algorithm, the present invention can greatly reduce the nonlinear control of the electromagnetic valve, speed up the response speed, so as to adapt a wider scale from 2% FS to 100% FS. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]      FIG. 1  is a block diagram of a prior art MFC. 
           [0045]      FIG. 2  is a schematic diagram of a closed-loop circuit according to the prior art MFC. 
           [0046]      FIG. 3  is a schematic diagram of an electromagnetic valve according to the prior art MFC. 
           [0047]      FIG. 4  is a schematic diagram illustrating a control mechanism of a MFC according to the first embodiment of the present invention. 
           [0048]      FIG. 5  is a block diagram of a MFC according to the first embodiment of the present invention. 
           [0049]      FIG. 6  is a flow sheet of a mass flow controlling method according to the first embodiment of the present invention. 
           [0050]      FIG. 7  is a block diagram of a MFC according to the second embodiment of the present invention. 
           [0051]      FIG. 8  is a flow sheet of a mass flow controlling method according to the second embodiment of the present invention. 
           [0052]      FIG. 9  is a flow sheet of a mass flow controlling method according to the third embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0053]    The present invention will be described in further details hereinafter by referring to the accompanying drawings, so as to provide a better understanding of the present invention. However, various modifications and variations can be made by the ordinary skilled in the art without departing the spirit and scope of the present invention. 
         [0054]    Referring to  FIG. 4 , where a control mechanism of a MFC according to the first embodiment of the present invention is shown. As shown in the  FIG. 4 , the MFC of the present invention adds a control part having an open-loop circuit compared to the prior art in the  FIG. 2 . A setting signal is firstly inputted by an input terminal, and converted into a first control voltage basing on a valve model  43  to be outputted to an electromagnetic valve  44 , so as to directly open the electromagnetic valve  44  to generate a fluid  45 . Next, a sensor  41  is used to sense a flow rate of the fluid  45  to generate and output a detection signal. A difference in the detection signal and the setting signal is used as a deviation value to proceed a calculation via a PID module  42 , so as to generate and output a second control voltage to the electromagnetic valve  44 . Finally, a superposition of the first control voltage and the second control voltage is used to regulate the openness of the electromagnetic valve  44 , so as to tightly control the flow rate of the fluid  45 . Wherein, the first control voltage is used to correct the nonlinearity of the electromagnetic valve, and the second control voltage is used for the PID control. 
         [0055]    The present invention will be further described in details hereinafter by referring to some embodiments. 
       The First Embodiment 
       [0056]    Referring to the  FIG. 5 , which is a block diagram of a MFC according to the first embodiment. The mass flow control apparatus comprises: an input terminal used to input a flow rate setting signal, a sensor unit  51  coupled to a fluid to sense its flow rate and output a flow rate detection signal, a control unit and an electromagnetic valve  52 . Wherein, the flow rate setting signal inputted by the input terminal and the flow rate detection signal outputted by the sensor unit  51  are characterized by voltage values in a wide voltage range respectively, e. g., in the range of 0-5V, which represent a setting flow and a detection flow corresponding to a full scale respectively. However, it should be noted that the flow rate setting signal may also be a digital signal, which will not be limited by the present invention. The control unit aims to generate an openness control signal, and output that to the electromagnetic valve  52 , so as to control the flow rate of the fluid, basing on the flow rate setting signal and the flow rate detection signal. The control unit includes an A/D converter  53 , a microprocessor  54  and a valve control circuit  55 . The A/D converter  53  is coupled to the sensor unit  51  and the microprocessor  54  for converting a flow rate detection signal AS 2  outputted by the sensor unit  51  into a second digital signal DS 2  outputted to the microprocessor  54 . In addition, the A/D converter  53  is also coupled between the input terminal and the microprocessor  54  for converting a flow rate setting signal AS 1  inputted by the input terminal into a first digital signal DS 1  outputted to the microprocessor  54 , when the flow rate setting signal AS 1  is an analog signal. The microprocessor  54  is coupled to the A/D converter for receiving the first digital signal DS 1  and the second digital signal DS 2 . However, it should be noted that, the flow rate setting signal not only could be an analog signal, such as the AS 1 , which need to be converted into a digital signal via the A/D converter, such as the DS 1 , but also could be a digital signal, such as the DS 1 , which is directly inputted to the microprocessor without through the A/D converter. Two control signals S 1  and S 2  are generated by the microprocessor  54  basing on the first digital signal DS 1  and the second digital signal DS 2 , and outputted to the valve control circuit  55 . Then openness control signals corresponding to the control signals S 1  and S 2  respectively are generated by the valve control circuit  55  and outputted for controlling the electromagnetic valve  52 . Finally, the flow rate of the fluid through the pipeline is precisely controlled. 
         [0057]    Referring to the  FIG. 5 , again. The microprocessor  54  comprises a control module  541  and a calculation module  542 . The control module  541  is used to receive the first digital signal DS 1  and generate the corresponding first control signal S 1 . Specifically, a valve model, which is used to characterize a correspondence relation between the first digital signal and the first control signal, and stored in a storage module  543 , is adopted to generate the first control signal S 1  basing on the first digital signal DS 1 . The storage module  543 , such as EEPROM, may be embedded into the microprocessor  54  (shown in the  FIG. 5 ), or external to the microprocessor  54 , which will not be limited by the present invention. The valve model may be obtained based on pre-collected data, for example, various first digital signals DS 1  and the corresponding first control signals S 1  are recorded in the form of a table. The valve model also may be fitted based on the pre-collected data by a fitting function, e. g., a piecewise function or a continuous function, which is used to describe the correspondence relation between the first digital signal and the first control signal. 
         [0058]    Referring to the  FIG. 5 , again. The calculation module  542  is used to generate a second control signal S 2  by running a calculation for a difference in the first digital signal DS 1  and the second digital signal DS 2  received. Specifically, a PID algorithm is applied to proceed the calculation to generate the second control signal S 2 . 
         [0059]    Referring to the  FIG. 5 , again. The valve control circuit  55  coupled to the microprocessor  54  comprises a first valve control circuit  551 , a second valve control circuit  552 , and a third valve control circuit  553 . The first valve control circuit  551  is used to receive the first control signal S 1  and generate a corresponding first openness control signal S 3 . The second valve control circuit  552  is used to receive the second control signal S 2  and generate a corresponding second openness control signal S 4 . The first valve control circuit  551  and the second valve control circuit  552  are designed by the means of D/A or PWM filtering to proceed a digital-to-analog conversion for the first control signal S 1  and the second control signal S 2  to generate the first openness control signal S 3  and the second openness control signal S 4 . The third valve control circuit  553  is coupled to the first valve control circuit  551  and the second valve control circuit  552 . When only the first openness control signal S 3  is received, the first openness control signal S 3  is outputted to the electromagnetic valve  52  to control its opening. When the first openness control signal S 3  and the second openness control signal S 4  are simultaneously received, the superposition of them is outputted to the opened electromagnetic valve  52  to control its openness. 
         [0060]    The flow control method for the mass flow control apparatus according to the present embodiment will be described in detail hereinafter by referring to specific examples. The flow rate setting signal is usually ranged from 0 to 5V, which is indicative of the setting flow rate of from 0 to 100% of a full scale. When a control voltage required by the electromagnetic valve is about 12V, the flow rate can cover a 100% full scale range, and the voltage value of the corresponding flow rate setting signal is 5V. It is assumed that the flow rate setting signal AS 1  is equal to 5V. The control method for using the mass flow control apparatus of the present invention in this embodiment is divided into two stages. 
         [0061]    At the first stage, the flow rate setting signal, a voltage value received by the input terminal, e. g., 5V, is converted into the corresponding first digital signal DS 1 - 5 V at an initial analog-to-digital conversion proportion via the A/D converter  53  (e. g., voltage values of 0-5V correspond to 16-bit digital signals of 0-65535, respectively.). The valve model fitted by a piecewise function, characterizes a correspondence relation between the first digital signal and the first control signal, e. g., when the first digital signal DS 1  corresponds to a setting flow rate of from 2% to 50% of a full scale, the received first control signal S 1  equals to a digital signal DS 1 ′- 2 V which corresponds to a voltage value of 2V; when the first digital signal DS 1  corresponds to a setting flow rate of from 50% to 100% of a full scale, the received first control signal S 1  equals to a digital signal DS 1 ′- 3 V which corresponds to a voltage value of 3V. Therefore, the first digital signal DS 1 - 5 V received by the control module  541  is converted into the corresponding first control signal DS 1 ′- 3 V for outputting to the first valve control circuit  551  basing on the valve model. Then, the first control signal DS 1 ′- 3 V outputted to the first valve control circuit  551  is converted into the first openness control signal S 3  with a control voltage value of 9V to be outputted to the third valve control circuit  553  for controlling the opening of the electromagnetic valve  52 , basing on an initial digital-to-analog conversion proportion (e. g., 16-bit digital signals of 0-65535 correspond to voltage values of 0-5V, respectively) and a correspondence relation between a flow rate voltage and an electromagnetic valve control voltage. Since the sensor unit  51  has not sensed the flow rate of the fluid in this process, the calculation module  542  and the second valve control circuit  553  remain in the not-operation state. 
         [0062]    At the second stage, after opening the electromagnetic valve  52 , a flow rate detection signal AS 2  with a voltage value of 3V is received by the sensor unit  51 , and converted into the second digital signal DS 2 - 3 V via the A/D converter  53  to be outputted to the calculation module  542  to execute a PID algorithm. Specifically, the calculation module  542  is used to generate the second control signal S 2  by running a proportional-integral-derivative (PID) control algorithm for a difference in the first digital signal DS 1 - 5 V and the second digital signal DK- 3 V. Then, the second control signal S 2  outputted to the second valve control circuit  552  is converted into the second openness control signal S 4  to be outputted to the third valve control circuit  553 , basing on an initial digital-to-analog conversion proportion (e. g., 16-bit digital signals of 0-65535 correspond to voltage values of 0-5V, respectively) and a correspondence relation between a flow rate voltage and an electromagnetic valve control voltage. Now, the first openness control signal S 3  and the second openness control signal S 4  are simultaneously received by the third valve control circuit  553 . So, the superposition of the first openness control signal S 3  and the second openness control signal S 4  is outputted to the opened electromagnetic valve to regulate its openness for regulating its flow rate. Then, the regulated flow rate of the fluid is sensed by the sensor unit again. The received detection signal is successively passed through the A/D converter, the calculation module, the second valve control circuit and the third valve control circuit, and is converted into another second openness control signal. The above process is repeated until the voltage value of the second openness control signal S 4  is equal to 3V. Finally, the required control voltage 12V applied to the electromagnetic valve  52  is satisfied. 
         [0063]    Accordingly, the flow rate setting signal received from the input terminal is converted into the first control signal based on the valve model via the microprocessor, which is outputted to the first valve control circuit to directly open the electromagnetic valve. Therefore, the nonlinear change of the electromagnetic force at the electromagnetic valve opening stage in the prior art is corrected. Then, the second control signal is generated by running a PID algorithm via the microprocessor. The first control signal and the second control signal are converted into the first openness control signal and the second openness control signal, respectively, the superposition of which is utilized to regulate the openness of the opened electromagnetic valve. In conclusion, the present invention finally generates two valve control voltages based on the two digital signals received by the microprocessor, one of which is used to directly open the electromagnetic valve to a certain openness, and another of which is generated by a PID algorithm, and is used to regulate the openness of the opened electromagnetic valve by superposition with the first valve control voltage. Therefore, the nonlinear change of the electromagnetic force at the electromagnetic valve opening stage is avoided, and the response speed is accelerated. In the embodiment, assuming that the corresponded voltage value of the flow rate setting signal is 5V, the corresponded voltage value of the generated first control signal is 3V, which generates a valve control voltage of 9V to open the electromagnetic valve to a corresponding openness, so that a flow rate of the fluid is generated. Then, a flow rate detection signal of 3V corresponding to the flow rate of the fluid and the flow rate setting signal of 5V are outputted to the calculation module to proceed a PID algorithm, so as to generate a second control signal. The second control signal is outputted to the second valve control circuit for generating a second openness control signal, which is used to regulate the openness of the opened electromagnetic valve by superposition with the initial valve control voltage of 9V. Finally, the required control voltage 12V applied to the electromagnetic valve  52  is satisfied. It should be noted that the voltage value of the first control signal generated by the flow rate setting signal of 5V can be arbitrary, as long as the control voltage of the electromagnetic valve generated by the first control signal is not greater than the maximum control voltage allowed by the openness of the electromagnetic valve. 
         [0064]    A method of mass flow controlling applied to a mass flow control apparatus in the embodiment, shown in the  FIG. 6 , comprising the steps of: 
         [0065]    S 601 , receiving a flow rate setting signal which is used as or converted into a first digital signal. 
         [0066]    In this step, if the flow rate setting signal inputted from the input terminal is an analog signal AS 1 , it is converted into the first digital signal DS 1  via the A/D converter. If the flow rate setting signal inputted from the input terminal is a digital signal, it is directly used as the first digital signal DS 1  without conversion. 
         [0067]    S 602 , generating a corresponding first control signal based on the first digital signal, and opening an electromagnetic valve based on the generated first control signal. 
         [0068]    In this step, the first digital signal DS 1  is converted into the corresponding first control signal S 1  based on a valve model via the control module of the microprocessor. The valve model is used to characterize a correspondence relation between the first digital signal DS 1  and the first control signal S 1 , which can be obtained based on pre-collected data, or fitted based on the pre-collected data by a fitting function which is a piecewise function or a continuous function. Then, the first control signal S 1  is outputted to the first valve control circuit for generating a first openness control signal S 3 , which is applied to the electromagnetic valve for opening it. 
         [0069]    S 603 , sensing a flow rate of a fluid and outputting a flow rate detection signal. 
         [0070]    In this step, after opening the electromagnetic valve by the first openness control signal S 3 , a fluid flows through the pipeline. Then, the sensor unit senses the flow rate of the fluid and outputs a flow rate detection signal AS 2 . 
         [0071]    S 604 , converting the flow rate detection signal into a second digital signal, and generating and outputting a second control signal by a calculation for a difference in the first digital signal and the second digital signal. 
         [0072]    In this step, the flow rate detection signal AS 2  is converted into a second digital signal DS 2  via the A/D converter. The calculation module of the microprocessor generates a second control signal S 2  by running a PID algorithm for a difference in the first digital signal DS 1  and the second digital signal DS 2 , and outputs it. 
         [0073]    S 605 , controlling an openness of the electromagnetic valve according to the first control signal and the second control signal. 
         [0074]    In this step, the second control signal S 2  is received by the second valve control circuit for generating a second openness control signal S 4 . Then, the second openness control signal S 4  is outputted to the third valve control circuit for regulating the openness of the electromagnetic valve by superposition with the received first openness control signal S 3 , so as to regulate the flow rate of the fluid. Thereafter, steps S 603  to S 605  are repeated until the flow rate of the fluid reaches the setting flow rate. 
         [0075]    In conclusion, the method and the apparatus of mass flow controlling according to the embodiment of the present invention firstly applies a control voltage to the electromagnetic valve for opening it to a certain openness before running a PID algorithm, and then the openness of the electromagnetic valve is further regulated by running a PID algorithm. Therefore, the nonlinear change of the electromagnetic force at the electromagnetic valve opening stage is avoided, and the response speed is accelerated. 
       The Second Embodiment 
       [0076]    The method and the apparatus of mass flow controlling according to the second embodiment of the present invention will be further described in details hereinafter by referring to  FIGS. 7-8 . 
         [0077]    Referring to the  FIG. 7 , which is a block diagram of a MFC according to the second embodiment of the present invention. The mass flow control apparatus comprises: an input terminal used to input a flow rate setting signal AS 1 , a sensor unit  71  coupled to a fluid to sense its flow rate and output a flow rate detection signal AS 2 , a control unit and an electromagnetic valve  72 . The control unit aims to generate an openness control signal, and output that to the electromagnetic valve  52 , so as to control the flow rate of the fluid, basing on the flow rate setting signal AS 1  and the flow rate detection signal AS 2 . The control unit includes an A/D converter  73 , a microprocessor  74  and a valve control circuit  75 . The A/D converter  73  is coupled between the input terminal and the microprocessor  74  for converting a flow rate setting signal AS 1  inputted by the input terminal into a first digital signal DS 1  outputted to the microprocessor  74 . In addition, the A/D converter  73  is also coupled to the sensor unit  71  for converting a flow rate detection signal AS 2  into a second digital signal DS 2  outputted to the microprocessor  74 . However, it should be noted that, the flow rate setting signal not only could be an analog signal, such as the AS 1 , which need to be converted into a digital signal via the A/D converter, such as the DS 1 , but also could be a digital signal, such as the DS 1 , which is directly inputted to the microprocessor without through the A/D converter. Two control signals S 1  and S 2  are generated by the microprocessor  74  basing on the first digital signal DS 1  and the second digital signal DS 2 , and outputted to the valve control circuit  75 . Then openness control signals corresponding to the control signals S 1  and S 2  respectively are generated by the valve control circuit  75  and outputted for controlling the electromagnetic valve  72 . Finally, the flow rate of the fluid through the pipeline is precisely controlled. 
         [0078]    Referring to the  FIG. 7 , again. The microprocessor  74  comprises a control module  741  and a calculation module  742 . The control module  741  is used to receive the first digital signal DS 1  and generate the corresponding first control signal S 1 . Specifically, a valve model, which is used to characterize a correspondence relation between the first digital signal and the first control signal, and stored in a storage module  743 , is adopted to generate the first control signal S 1  basing on the first digital signal DS 1 . The storage module  743 , such as EEPROM, may be embedded into the microprocessor  74  (shown in the  FIG. 7 ), or external to the microprocessor  74 , which will not be limited by the present invention. The valve model may be obtained based on pre-collected data, for example, various first digital signals DS 1  and the corresponding first control signals S 1  are recorded in the form of a table. The valve model also may be fitted based on the pre-collected data by a fitting function, e. g., a piecewise function or a continuous function, which is used to describe the correspondence relation between the first digital signal and the first control signal. 
         [0079]    Referring to the  FIG. 7 , again. The calculation module  742  is used to generate a second control signal S 2  by running a calculation (e. g., a PID algorithm) for an adjusted difference in the first digital signal DS 1  and the second digital signal DS 2  by a first coefficient P 1 . The calculation module  742  is also coupled to the storage module  743  for calculating the first coefficient P 1 , which is a ratio of the first digital signal and a difference in the first digital signal and the first control signal, based on a valve model of the storage module  743 . Before running a PID algorithm, the difference D in the first digital signal and the first control signal is adjusted to D′ by proportion of the first coefficient P 1 , i. e. D′=D×P 1 . If the flow rate setting signal AS 1  ranged from 0 to AS 1 max is converted into a M-bit digital signal DS 1  at an initial analog-to-digital conversion proportion P AD  via the A/D converter, each digital signal corresponds a voltage range of AS 1 max/2 M , that is, the resolutions of the difference D is AS 1 max/2 M . After the difference D is adjusted by the first coefficient P 1 , each digital signal corresponds a voltage range of (AS 1 −voltage value corresponded by S 1 )/2 M , that is, the resolution of the adjusted difference D is changed into the (AS 1 −voltage value corresponded by S 1 )/2 M . Therefore, the resolution of the second signal S 2  generated by running a PID algorithm is improved, and the stability for controlling the electromagnetic valve is increased. 
         [0080]    Referring to the  FIG. 7 , again. The valve control circuit  75  coupled to the microprocessor  74  is used to generate an analog first openness control signal S 3  based on the first control signal S 1 , and generate an analog second openness control signal S 4  based on the second control signal S 2  and the first coefficient P 1 . Specifically, the valve control circuit  75  comprises a first valve control circuit  751 , a second valve control circuit  752 , and a third valve control circuit  753 . The first valve control circuit  751  is used to receive the first control signal S 1  and convert it into an analog voltage V 1  based on the proportion P DA  of the initial digital-to-analog conversion, i. e., V 1 =S 1 ×P DA , which is further converted into a corresponding first openness control signal S 3 . The second valve control circuit  752  is used to receive the second control signal S 2  and convert it into an analog voltage V 2  based on the first coefficient P 1  and the proportion of the initial digital-to-analog conversion P DA , i. e., V 2 =S 2 ×P DA /P 1 , which is further converted into a corresponding second openness control signal S 4 . The first valve control circuit  751  and the second valve control circuit  752  are designed by the means of D/A or PWM filtering to proceed a digital-to-analog conversion for the first control signal S 1  and the second control signal S 2 . The first coefficient P 1  can be received from the microprocessor, or directly received from the valve model by the valve control circuit. The third valve control circuit  753  is coupled to the first valve control circuit  751  and the second valve control circuit  752 . When only the first openness control signal S 3  is received, the first openness control signal S 3  is outputted to the electromagnetic valve  72  to control its opening. When the first openness control signal S 3  and the second openness control signal S 4  are simultaneously received, the superposition of them is outputted to the opened electromagnetic valve  72  to control its openness. 
         [0081]    The flow control method for the mass flow control apparatus according to the present embodiment will be described in detail hereinafter by referring to specific examples. It is assumed that the flow rate setting signal AS 1  is equal to 4.5V, and the corresponding control voltage of the electromagnetic valve is equal to 11V. The control method for using the mass flow control apparatus of the present invention in this embodiment is divided into two stages. 
         [0082]    At the first stage, the flow rate setting signal, a voltage value received by the input terminal, e. g., 4.5V, is converted into the corresponding first digital signal DS 1 - 4 . 5 V at an initial analog-to-digital conversion proportion via the A/D converter  73  (e. g., voltage values of 0-5V correspond to 16-bit digital signals of 0-65535, respectively.). The valve model fitted by a piecewise function, characterizes a correspondence relation between the first digital signal and the first control signal, e. g., when the first digital signal DS 1  corresponds to a setting flow rate of from 2% to 50% of a full scale, the received first control signal S 1  equals to a digital signal DS 1 ′- 2 V which corresponds to a voltage value of 2V; when the first digital signal DS 1  corresponds to a setting flow rate of from 50% to 100% of a full scale, the received first control signal S 1  equals to a digital signal DS 1 ′- 3 V which corresponds to a voltage value of 3V. Therefore, the first digital signal DS 1 - 4 . 5 V received by the control module  741  is converted into the corresponding first control signal DS 1 ′- 3 V for outputting to the first valve control circuit  751  based on the valve model. Then, the first control signal S 1 =DS 1 ′- 3 V is converted into an analog voltage V 1  based on the initial digital-to-analog conversion proportion P DA  (e. g., 16-bit digital signals of 0-65535 correspond to voltage values of 0-5V, respectively) via the first valve control circuit  751 , i. e., V 1 =DS 1 ′3 v×P DA , which is further converted into the corresponding first openness control signal S 3  with a control voltage value of 9V to be outputted to the third valve control circuit  753  for controlling the opening of the electromagnetic valve  72 . Since the sensor unit  71  has not sensed the flow rate of the fluid in this process, the calculation module  742  and the second valve control circuit  753  remain in the not-operation state. 
         [0083]    At the second stage, after opening the electromagnetic valve  72 , a flow rate detection signal AS 2  with a voltage value of 3V is received by the sensor unit  71 , and converted into the second digital signal DS 2 - 3 V via the A/D converter  73  to be outputted to the calculation module  742  to execute a PID algorithm. Specifically, the calculation module  742  is firstly used to adjust the difference D in the first digital signal DS 1 - 4 . 5 V and the second digital signal DS 2 - 3 V by proportion of the first coefficient P 1 , i. e., the adjusted difference D′=D×4.5/(4.5−3), of which the resolution is equal to 1.5V/65534, significantly improved comparing to the initial resolution 5V/65535. Then, the second control signal S 2  is generates by running a proportional-integral-derivative (PID) algorithm for the adjusted difference D′ by the calculation module  742 , which resolution is also improved. After the generated second control signal S 2  is outputted to the second valve control circuit  752  via the calculation module  742 , the analog voltage V 2  of the second control signal S 2  is firstly generated based on the second control signal S 2  and the adjusted digital-to-analog conversion proportion P DA ′, i. e., V 2 =S 2 ×P DA ′, wherein, the adjusted digital-to-analog conversion proportion P DA ′ is a ratio of the initial digital-to-analog conversion proportion P DA  (e. g., 16-bit digital signals of 0-65535 correspond to voltage values of 0-5V, respectively) and the first coefficient P 1 , i. e., P DA ′=P DA /P 1 . Then, the analog voltage V 2  is converted into the corresponding valve control voltage of the second openness control signal S 4  to be outputted to the third valve control circuit  753 . Now, the first openness control signal S 3  and the second openness control signal S 4  are simultaneously received by the third valve control circuit  753 . So, the superposition of them is outputted to the opened electromagnetic valve  72  to further regulate its openness. Finally, the second openness control signal S 4  applied to the electromagnetic valve  72  is repeatedly adjusted until the valve control voltage of it reaches the value of 11V. 
         [0084]    A method of mass flow controlling applied to a mass flow control apparatus in the embodiment, shown in the  FIG. 8 , comprising the steps of: 
         [0085]    S 801 , receiving a flow rate setting signal which is used as or converted into a first digital signal. 
         [0086]    In this step, if the flow rate setting signal inputted from the input terminal is an analog signal AS 1 , it is converted into the first digital signal DS 1  at the proportion P AD  of the initial analog-to-digital conversion via the A/D converter. If the flow rate setting signal inputted from the input terminal is a digital signal, it is directly used as the first digital signal DS 1  without conversion. 
         [0087]    S 802 , generating a corresponding first control signal based on the first digital signal, and outputting it. 
         [0088]    In this step, the first digital signal DS 1  is converted into the corresponding first control signal S 1  based on a valve model via the control module of the microprocessor. The valve model can be obtained based on pre-collected data, or fitted as a piecewise function or a continuous function based on the pre-collected data. 
         [0089]    S 803 , generating a first openness control signal basing on the first control signal, and opening an electromagnetic valve basing on the first openness control signal. 
         [0090]    In this step, the first control signal S 1  is firstly converted into an analog signal by the initial digital-to-analog conversion proportion P DA  (i. e. the reciprocal of the analog-to-digital conversion proportion P AD ) via the first valve control circuit. And then the corresponding first openness control signal S 3  is generated for opening the electromagnetic valve. 
         [0091]    S 804 , sensing a flow rate of a fluid and outputting a flow rate detection signal. 
         [0092]    In this step, after opening the electromagnetic valve by the first openness control signal S 3 , a fluid flows through the pipeline. Then, the sensor unit senses the flow rate of the fluid and outputs a flow rate detection signal AS 2 . 
         [0093]    S 805 , converting the flow rate detection signal into a second digital signal, and generating a second control signal by a calculation for an adjusted difference in the first digital signal and the second digital signal by a first coefficient, which is a ratio of the first digital signal and a difference in the first digital signal and the first control signal. 
         [0094]    In this step, the flow rate detection signal AS 2  is firstly converted into a second digital signal DS 2  at the proportion P AD  of the initial analog-to-digital conversion via the A/D converter. Then, the difference D of the first digital signal DS 1  and the second digital signal DS 2  is adjusted by the first coefficient P 1 , i. e., the adjusted difference D′=D×P 1 . Wherein, the first coefficient P 1  is a ratio of the first digital signal and a difference in the first digital signal and the first control signal, i. e., P 1 =DS 1 /(DS 1 −S 1 ). Finally, the second control signal S 2  is generated by running a PID algorithm for the adjusted difference D′, and outputted. The resolution of the second control signal S 2  is calculated by a formula of (AS 1 −voltage value corresponded by the S 1 )/2 M . 
         [0095]    S 806 , generating a second openness control signal according to the second control signal and the first coefficient, and controlling an openness of the electromagnetic valve according to the first openness control signal and the second openness control signal. 
         [0096]    In this step, the second control signal S 2  is firstly converted into an analog voltage V 2  based on the initial digital-to-analog conversion proportion P DA  and the first coefficient P 1 , i. e., V 2 =S 2 ×P DA /P 1 . Then, the corresponding second openness control signal S 4  is generated and outputted to the third valve control circuit  753  for further regulating the openness of the opened electromagnetic valve  72  by superposition with the first openness control signal S 3 . Finally, the steps S 804 -S 806  are repeated until the flow rate of the fluid reaches the required flow rate. 
         [0097]    Comparing to the first embodiment, the resolution of the difference of the received first digital signal and second digital signal by the calculation module is improved by the present embodiment, so that the resolution and accuracy of the second control signal are improved, and the stability of the second openness control signal to the control of the electromagnetic valve is increased. 
       The Third Embodiment 
       [0098]    The present embodiment further improves the resolution and the accuracy of the first control signal on the base of the second embodiment. 
         [0099]    The mass flow control apparatus of the present embodiment comprises: an input terminal, a sensor unit  71 , a control unit and an electromagnetic valve  72 . The control unit includes an A/D converter  73 , a microprocessor  74  and a valve control circuit  75 . The A/D converter  73 , the sensor unit  71 , and the input terminal play the same roles as the above-mentioned embodiments, so the detail description for them will be omitted. 
         [0100]    The microprocessor  74  comprises a control module  741  and a calculation module  742 . The control module  741  is used to receive the first digital signal DS 1 , which is not directly converted into the first control signal, but firstly converted into a corresponding decomposition signal DS 1 ′. Then, the first control signal S 1  is generated by adjusting the decomposition signal DS 1 ′ with a proportion. Specifically, a valve model is employed by the control module  741  to convert the first digital signal DS 1  into the decomposition signal DS 1 ′, which is smaller than the first digital signal DS 1 . The valve model is stored in a storage module  743 , such as EEPROM, and is used to characterize a correspondence relation between the first digital signal and the decomposition signal. The valve model may be obtained based on pre-collected data, for example, various first digital signals DS 1  and the corresponding decomposition signals DS 1 ′ are recorded in the form of a table. The valve model also may be fitted as a piecewise function or a continuous function based on the pre-collected data, which is used to describe the correspondence relation between the first digital signal DS 1  and the decomposition signal DS 1 ′. 
         [0101]    The control module  741  is used to generate a first control signal S 1  by adjusting the decomposition signal DS 1 ′ with a second coefficient P 2 , i. e., S 1 =DS 1 ′×P 2 . Wherein, the second coefficient P 2  is calculated based on the valve model of the control module  741 , which is a ratio of the first digital signal and the decomposition signal, i. e., P 2 =DS 1 /DS 1 ′. If the flow rate setting signal AS 1  ranged from 0 to AS 1 max is converted into a M-bit digital signal DS 1  at an initial analog-to-digital conversion proportion P AD  via the A/D converter, each digital signal corresponds a voltage range of AS 1 max/2 M , that is, the resolutions of the first digital signal is AS 1 max/2 M . But, after the decomposition signal DS 1 ′ is adjusted by the second coefficient P 2 , each digital signal corresponds a voltage range of AS 1 max/2 M  (AS 1 max′ is the max voltage value corresponded by the adjusted decomposition signal). Due to DS 1 ′&lt;DS 1 , the resolution of the generated first control signal S 1  is improved. 
         [0102]    The calculation module  742  is used to generate a second control signal S 2  by running a calculation (e. g., a PID algorithm) for an adjusted difference in the first digital signal DS 1  and the second digital signal DS 2  by a first coefficient P 1 . The calculation module  742  is also coupled to the storage module  743  for calculating the first coefficient P 1 , which is a ratio of the first digital signal DS 1  and a difference in the first digital signal DS 1  and the decomposition signal DS 1  ‘, i. e., P 1 =DS 1 /(DS 1 −DS 1 ’), based on a valve model. Before running a PID algorithm, the difference D in the first digital signal and the first control signal is adjusted to D′ by proportion of the first coefficient P 1 , i. e. D′=D×P 1 . At this time, each digital signal corresponds a voltage range of (AS 1 max−AS 1 max′)/2 M . Therefore, the resolution of the generated second control signal S 2  is improved. 
         [0103]    The valve control circuit  75  coupled to the microprocessor  74  is used to generate an analog first openness control signal S 3  based on the first control signal S 1  and the second coefficient P 2 , and generate an analog second openness control signal S 4  based on the second control signal S 2  and the first coefficient P 1 . Specifically, the valve control circuit  75  comprises a first valve control circuit  751 , a second valve control circuit  752 , and a third valve control circuit  753 . The first valve control circuit  751  is used to receive the first control signal S 1  and convert it into an analog voltage V 1  based on the second coefficient P 2  and the proportion P DA  of the initial digital-to-analog conversion, i. e., V 1 =S 1 ×P DA /P 2 , which is further converted into a corresponding first openness control signal S 3 . The second valve control circuit  752  is used to receive the second control signal S 2  and convert it into an analog voltage V 2  based on the first coefficient P 1  and the proportion P DA  of the initial digital-to-analog conversion, i. e., V 2 =S 2 ×P DA /P 1 , which is further converted into a corresponding second openness control signal S 4 . The first valve control circuit  751  and the second valve control circuit  752  are designed by the means of D/A or PWM filtering to proceed a digital-to-analog conversion for the first control signal S 1  and the second control signal S 2 . The first coefficient P 1  and the second coefficient P 2  can be received from the microprocessor, or directly received from the valve model by the valve control circuit. The third valve control circuit  753  is coupled to the first valve control circuit  751  and the second valve control circuit  752 . When only the first openness control signal S 3  is received, the first openness control signal S 3  is outputted to the electromagnetic valve  72  to control its opening. When the first openness control signal S 3  and the second openness control signal S 4  are simultaneously received, the superposition of them is outputted to the opened electromagnetic valve  72  to control its openness. 
         [0104]    A method of mass flow controlling applied to a mass flow control apparatus in the embodiment, shown in the  FIG. 9 , comprising the steps of: 
         [0105]    S 901 , receiving a flow rate setting signal AS 1  which is used as or converted into a first digital signal DS 1 . 
         [0106]    In this step, if the flow rate setting signal inputted from the input terminal is an analog signal AS 1 , it is converted into a M-bit first digital signal DS 1  at the proportion P AD  of the initial analog-to-digital conversion via the A/D converter. If the flow rate setting signal inputted from the input terminal is a digital signal, it is directly used as the first digital signal DS 1  without conversion. 
         [0107]    S 902 , generating a corresponding decomposition signal basing on the first digital signal. 
         [0108]    In this step, the first digital signal DS 1  is converted into the corresponding decomposition signal DS 1 ′ based on a valve model via the control module of the microprocessor. The valve model can be obtained based on pre-collected data, or fitted as a piecewise function or a continuous function based on the pre-collected data. It is preferable that the decomposition signal DS 1 ′ is smaller than the first digital signal DS 1 . 
         [0109]    S 903 , generating a first control signal by adjusting the decomposition signal with a second coefficient. 
         [0110]    In this step, the first control signal S 1  is calculated by a formula of S 1 =DS 1 ′×P 2 , wherein, the second coefficient P 2  is calculated by a formula of P 2 =DS 1 /DS 1 ′. The resolution of the first control signal S 1  is equal to AS 1 ′/2 M , wherein, the AS 1 ′ is a voltage value converted by the decomposition signal DS 1 ′ with a proportion P DA  of the initial digital-to-analog conversion. 
         [0111]    S 904 , generating a first openness control signal based on the first control signal and the second coefficient, and opening an electromagnetic valve based on the first openness control signal. 
         [0112]    In this step, the first control signal S 1  is firstly converted into an analog signal V 2 , which is further converted into the corresponding first openness control signal S 3 , by the initial digital-to-analog conversion proportion P DA  and the second coefficient P 2  via the first valve control circuit, i. e., V 2 =S 3 =S 1 ≦P DA /P 2 . And then the first openness control signal S 3  is applied to the electromagnetic valve by the first valve control circuit for opening it. 
         [0113]    S 905 , sensing a flow rate of a fluid and outputting a flow rate detection signal. 
         [0114]    In this step, after opening the electromagnetic valve by the first openness control signal S 3 , a fluid flows through the pipeline. Then, the sensor unit senses the flow rate of the fluid and outputs a flow rate detection signal AS 2 . 
         [0115]    S 906 , converting the flow rate detection signal into a second digital signal, and generating a second control signal by a calculation for an adjusted difference in the first digital signal and the second digital signal by a first coefficient, which is a ratio of the first digital signal and a difference in the first digital signal and the decomposition signal. 
         [0116]    In this step, the flow rate detection signal AS 2  is firstly converted into a second digital signal DS 2  at the proportion P AD  of the initial analog-to-digital conversion via the A/D converter. Then, the difference D of the first digital signal DS 1  and the second digital signal DS 2  is adjusted by the first coefficient P 1 , i. e., the adjusted difference D′=D×P 1 . Wherein, the first coefficient P 1  is a ratio of the first digital signal and a difference in the first digital signal and the decomposition signal, i. e., P 1 =DS 1 /(DS 1 −DS 1 ′). Finally, the second control signal S 2  is generated by running a PID algorithm for the adjusted difference D′, and outputted. The resolution of the second control signal S 2  is calculated by a formula of. Finally, the second control signal S 2  is generated by running a PID algorithm for the adjusted difference D′, and outputted. 
         [0117]    The resolution of the second control signal S 2  is calculated by a formula of (AS 1 −AS 1 ′)/2 M . 
         [0118]    S 907 , generating a second openness control signal according to the second control signal and the first coefficient, and controlling an openness of the electromagnetic valve according to the first openness control signal and the second openness control signal. 
         [0119]    In this step, the second control signal S 2  is firstly converted into an analog voltage V 2  based on the initial digital-to-analog conversion proportion P DA  and the first coefficient P 1 , i. e., V 2 =S 2 ×P DA /P 1 . Then, the corresponding second openness control signal S 4  is generated and outputted to the third valve control circuit  753  for further regulating the openness of the opened electromagnetic valve  72  by superposition with the first openness control signal S 3 . Finally, the steps S 905 -S 907  are repeated until the flow rate of the fluid reaches the required flow rate. 
         [0120]    Due to improving the resolutions of the first control signal and the second control signal, the stability and the accuracy for controlling the electromagnetic valve in the present embodiment are increased. 
         [0121]    In summary, the present invention generates two control signal, one of which is the first control signal generated by the flow rate setting signal for directly opening the electromagnetic valve to a certain openness, another of which is the second control signal generated by the flow rate setting signal and the flow rate detection signal for regulating the openness of the opened electromagnetic valve by superposition with the first control signal. That is, to further regulate the openness of the electromagnetic valve by running a PID algorithm after opening the electromagnetic valve to certain openness. Therefore, the nonlinear change of the electromagnetic force at the electromagnetic valve opening stage is avoided, and the response speed is accelerated. 
         [0122]    Although the present invention has been disclosed as above with respect to the preferred embodiments, they should not be construed as limitations to the present invention. Various modifications and variations can be made by the ordinary skilled in the art without departing the spirit and scope of the present invention. Therefore, the protection scope of the present invention should be defined by the appended claims.