A mass air flow measurement circuit is disclosed which provides a digital output without using a separate analog to digital (A/D) converter. This air flow measurement circuit includes a bridge circuit, wherein voltage is supplied to the bridge to control current through one or more sense elements in order to keep the temperature and resistances of the sense elements constant and balance the bridge. An electrical comparator is used to sense the balance of the bridge, and the output of the comparator is applied to an input port of a microcomputer. The microcomputer generates a rectangular wave pulse width modulated (PWM) signal whose duty cycle is increased or decreased in response to the output of the comparator. The PWM signal is then amplified and applied to the bridge to balance the bridge. Mass air flow is then calculated by multiplying the duty cycle of the PWM signal by a proportionally constant.
An improved electronic mass airflow sensor circuit and method for manufacturing that circuit are disclosed. The circuit includes primary and secondary sensor elements having non-zero temperature coefficients of resistance. The circuit further includes a thick film resistor electrically connected to the secondary sensor element to form a combination element such that resistance ratio of the primary sensor element to the combination element remains constant over varying temperatures. The method includes the steps of detecting the resistance of the primary and secondary sensor elements, generating primary and secondary electrical signals corresponding thereto, and placing the thick film resistor in electrical contact with the secondary sensor element to form the combination element. The method also includes the steps of processing the primary and secondary electrical signals to determine operating temperatures for the primary sensor element and an optimum resistance for the thick film resistor such that the resistance ratio of the primary sensor element to the combination element remains constant over varying temperatures. The method further includes the step of sizing the thick film resistor in accordance with the optimum resistance determined.
A time domain measurement and control system for a hot wire air flow sensor is disclosed in which the air flow sensor is of the type having a resistive heating element with an input end and an output end. The control system includes a fixed frequency variable width pulse generator which generates a pulse train through the heating element. This pulse train, furthermore, has a first predetermined voltage amplitude. The output voltage amplitude of the pulse train is determined at the outlet end of the heating element while a control circuit varies the duty cycle of the pulse train to maintain the output voltage amplitude at a second predetermined voltage level. The duty cycle of the pulse train is proportional to the air flow rate through the air flow sensor. In a modification of the invention, a fixed width variable frequency pulse train is generated through the resistive heating element and the frequency of the pulse train is varied to maintain the output voltage amplitude of the pulse train at the outlet end of the resistive heating element at a second predetermined voltage level. In this modification, the frequency of the pulse train is proportional to the air flow rate through the air flow sensor.
A flow meter includes a body located in thermal contact with a fluid, the flow rate of which is to be measured, a heater in thermal contact with the body, a first temperature sensor for measuring the temperature of the body, and a second temperature sensor for measuring the temperature of the fluid. The flow meter also includes mechanisms for calculating the differential temperature between the fluid and the body, for supplying power to the heater, for measuring the rate at which power is supplied to the heater, for setting a predetermined target differential temperature, and predetermined maximum and minimum differential temperatures, and for comparing the calculated differential temperature with the predetermined target, maximum and minimum differential temperatures. The flow meter further includes mechanisms for controlling the rate that power is supplied to the heater and for calculating the flow rate from the measured rate of power supply and the difference between the temperatures of the body and the fluid.
The velocity of a fluid in a positive or negative direction is measured by se of a thermistor in a feedback circuit. The temperature of a flow thermistor is balanced with the temperature of a reference thermistor by sensing temperature through resistance by way of its voltage and adding power until the flow thermistor is at the same temperature as the reference thermistor. The ratio of reference power used as compared to flow power is indicative of the fluid's speed. Power is added by sending a signal of varying frequency to the flow thermistor; the reference thermistor is powered by a constant frequency signal. Power is measured by comparing the frequencies of the reference signal to the flow signal. Fluid direction is measured by placing a direction thermistor on either side of the flow thermistor, measuring the temperature of each direction thermistor, the cooler thermistor being upstream of the flow thermistor.
A method and a device serve for measuring fluidic or calorimetric parameters, for example the velocity of flow, and comprise the steps of heating up a sensor element arranged in a measuring environment, to a temperature higher than the surrounding temperature of the measuring environment, and deriving the parameter from the heat transmission between the measuring environment and the sensor element. In order to permit measurements the result of which is systematically independent of the absolute value of the heating power in the sensor element, two measurements are carried out successively at different temperatures, in the stationary condition. One determines the heating power at both these operating conditions and derives therefrom the parameter to be measured (FIG. 7).
