Mass flow rate-controlling apparatus

1. A mass flow rate controlling apparatus comprising a mass flow rate-detecting means attached to a fluid flow path, through which a fluid is supplied, for detecting a mass flow rate at predetermined time intervals, a flow rate-controlling valve mechanism attached to said fluid flow path for changing a valve-opening degree in response to a valve operating signal to control said mass flow rate, a controlling means for supplying said valve-operating voltage to said flow rate controlling valve mechanism, and a pressure-detecting means attached to said fluid flow path for detecting the pressure of said fluid at predetermined time intervals;

wherein a pressure variation is obtained from the detected pressure;

wherein said controlling means uses a first control mode for controlling said mass flow rate when said pressure variation is less than a predetermined threshold value;

wherein in said first control mode, said controlling means stores a newest detected mass flow rate, a newest detected pressure and a newest valve-operating voltage for a current time interval;

wherein in said first control mode, said controlling means compares a present mass flow rate signal obtained from said mass flow rate-detecting means with an externally input flow rate-setting signal to determine said valve-operating voltage;

wherein said controlling means switches said first control mode to a second control mode when said pressure variation becomes equal to or more than said predetermined threshold value; and

wherein in said second control mode, said controlling means determines said valve-operating voltage from the newest mass flow rate, the newest detected pressure and the newest valve operating voltage stored in said first control mode, a currently detected pressure, and valve characteristics stored in advance.

2. The mass flow rate-controlling apparatus according to claim 1 , wherein said valve-operating voltage Vn in said second control mode is determined by the following formula (1);

Vn=Vb+ (1 −Pb/Pn )/ k   (1),

wherein Vb is the newest valve-operating voltage, Pb is the newest detected pressure, Pn is the currently detected pressure, and k is a flow rate variation ratio per a unit volt at the valve-operating voltage before the pressure variation becomes equal to or more than the predetermined threshold value.

3. The mass flow rate-controlling apparatus according to claim 1 , wherein said controlling means outputs a mass flow rate before said pressure variation reaches said predetermined threshold value as a flow rate output signal in said second control mode.

4. The mass flow rate-controlling apparatus according to claim 1 , wherein said fluid path is provided with said pressure-detecting means, said mass flow rate-detecting means and said flow rate-controlling valve mechanism in this order from the upstream side.

5. The mass flow rate-controlling apparatus according to claim 1 , wherein said fluid path is provided with said mass flow rate-detecting means, said pressure-detecting means and said flow rate-controlling valve mechanism in this order from the upstream side.

6. The mass flow rate-controlling apparatus according to claim 1 , wherein said flow rate-controlling valve mechanism comprises an actuator comprising a piezoelectric element.

7. The mass flow rate-controlling apparatus according to claim 1 , wherein said flow rate-controlling valve mechanism comprises an electromagnetic actuator.

8. The mass flow rate-controlling apparatus according to claim 1 , wherein said fluid path comprises an orifice in the most upstream portion.

9. The mass flow rate-controlling apparatus according to claim 1 , wherein said fluid path comprises an orifice in the most downstream portion.

10. The mass flow rate-controlling apparatus according to claim 1 , wherein said controlling means comprises RAM and ROM in combination.