Method for calibrating the time response of a mass air flow sensor by laser trimming selected resistors and without an air flow
AssigneeFord Motor Company
InventorJames M. Sherman
Life of this patent
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Abstract
A method for calibrating the response time of a mass air flow sensor in which the sensors bridge offset voltage is measured and the constant C.sub.I is calculated from the initial value of the bridge offset voltage. C.sub.I is a constant of the heated resistor's resistance in the equation RH=C+(1/I.sub.H) C.sub.I. The response time of the mass air flow sensor is a function of C.sub.I. A bridge offset voltage required to give C.sub.I a desired value is calculated. A pair of biasing resistors are trimmed while monitoring the value of the bridge offset voltage until the monitored value of the bridge offset voltage is equal to the desired value.
Claims12
A method for calibrating the response time of a mass air flow sensor having a heated resistor, a cold resistor and a balanced bridge circuit having a bridge offset voltage V.sub.br, a reference voltage V.sub.REF and a first and second biasing resistor connected to said reference voltage affecting the value of said bridge offset voltage, said method comprising the steps of:
- measuring a value of said bridge offset voltage;
- calculating a value of a constant C.sub.I of the heated resistor related to the response time of the balanced bridge circuit from said value of said bridge offset voltage;
- calculating a value of a required bridge offset voltage necessary to generate a desired value of said constant C.sub.I which would produce a response time for said mass air flow sensor within specified limits;
- comparing said value of said bridge offset voltage to said value of said required bridge offset voltage to determine which of said bridge offset voltage and said required bridge offset voltage has the greater value;
- laser trimming said first biasing resistor to decrease said value of said bridge offset voltage to said value of said required bridge offset voltage in response to said bridge offset voltage having a value greater than said required bridge offset voltage;
- and
- laser trimming said second biasing resistor to increase said value of said bridge offset voltage to said value of said required bridge offset voltage in response to said required bridge offset voltage having a value greater than said bridge offset voltage.
A method for calibrating the response time of a mass air flow sensor having a heated resistor, a cold resistor and a balanced bridge circuit, said balanced bridge circuit comprising an operational amplifier having a positive input, a negative input and an output connected to said heated resistor and said cold resistor, said circuit further having a first biasing resistor connecting the positive input of said operational amplifier to a reference voltage and a second biasing resistor connecting said negative input of said operational amplifier to said reference voltage, the resistance values of said first and second biasing resistors controlling a bridge offset voltage generated by the current flow through the heated resistor and the cold resistor, said method comprising the steps of:
- applying electrical power to said mass air flow sensor;
- measuring an initial value of said bridge offset voltage;
- calculating an initial value of a constant C.sub.I of the resistance of said heated resistor from said value of said bridge offset voltage where said heated resistor has a value RH=C+(1/I.sub.H) C.sub.I, and where:
- C is a constant equal to the approximated heated resistance value of said heated resistor controlled by said balanced bridge circuit,
- C.sub.I is a small constant controlling the response time of the mass air flow sensor, and
- I.sub.H is the current through the heated resistor,
- calculating a required value for said bridge offset voltage required to generate a predetermined value of said constant C.sub.I, said predetermined value of said constant C.sub.I selected to give said mass air flow sensor a desired response time;
- comparing said required value for said bridge offset voltage with said value of said bridge offset voltage to determine which has the greater value;
- laser trimming said first biasing resistor to decrease said value of said bridge offset voltage to be equal to said required value in response to said bridge offset voltage having a value greater than said required value;
- and
- laser trimming said second biasing resistor to increase said value of said bridge offset voltage to be equal to said required value in response to said required value of said bridge offset voltage being greater than said value of said bridge offset voltage.
Dependencies are inferred from the claim text.