Hitachi, Ltd.

Hitachi Automotive Systems, Ltd.

Ryo Ando

Takeo Hosokawa

Hiroshi Onuki

Shigenobu Komatsu

Hiroshi Nakano

Masuo Akamatsu

Daisuke Terada

Masayuki Hio

Yoshiyuki Akiyama

According to the conventional knowledge, a thermal humidity measurement device has problems in which the direction of air flow is limited to one direction is too hard to handle in securing the humidity responsiveness because there are limitations on the mounting direction of the thermal humidity measurement device and the state of air flow. An introduction guide protrudes from an air introduction surface to the outside of a measurement chamber, is parallel to a humidity introduction port surface, and has a portion not in contact with an inlet surface of a humidity introduction tube as seen from any direction to guide the flow of air to the humidity introduction tube from any direction of 360&#xb0;.

Humidity measurement device

A thermal humidity measuring device includes first and second heating elements, and is capable of obtaining a plurality of measurement values (measured values) by effectively utilizing each of the heating elements, which includes measuring humidity by the first heating element. A thermal humidity measuring device includes a first bridge circuit that includes a first heating element that senses humidity, and a second bridge circuit that includes a second heating element that heats air around the first heating element. In the thermal humidity measuring device, a first output signal is extracted from the first bridge circuit, and the humidity is sensed. A second output signal is extracted from the second bridge circuit, and the second output signal includes information relating to at least any one of pressure, an air flow rate, and air temperature.

Thermal humidity measuring device

A thermal-type flow meter for representing a flow rate of air by the frequency of a periodic signal, wherein abnormalities in the waveform of an output signal due to frequency variation is prevented while high-frequency noise is suppressed. The thermal-type flow meter pertaining to the present invention is provided with a plurality of switching elements connected in parallel, and varies a delay width between the switching elements in accordance with variation of the frequency of a periodic signal for representing a flow rate.

Thermal type flow meter

Unwanted electromagnetic waves are eliminated in a frequency output circuit comprising an open-collector output terminal whereby radio noise can be reduced. A frequency output circuitcomprises an open-collector transistor Qwith an open-collector output terminal connected to a pull-up resistor Rf connected to an external power supply Vex. A current limiting circuitlimits a current through the output terminal such that a high potential and a low potential of the output waveform of a frequency signal outputted by an output circuitare smoothly switched. The current limiting circuitis formed by a differential pair circuit of transistors Qand Qeach with a resistor provided in the emitter thereof, and a current mirror circuit comprising an open-collector transistor, for example.

Frequency output circuit

The time it takes for a hot-wire airflow meter to begin to operate normally upon startup of an automobile is reduced. A heat-generating resistor Rh for measuring airflow rate, a temperature-compensating resistor Rc, and a resistor Rfor detecting the current through the heat-generating resistor are connected in parallel. A differential voltage between a power supply voltage Vb and the voltage across the emitter-collector junction of transistors Q, Q, or Qis applied to each of the heat-generating resistor Rh, temperature-compensating resistor Rc, and current-detecting resistor R. The transistors Q, Q, and Qform a current mirror circuit, by which the ratio of currents through the heat-generating resistor Rh, temperature-compensating resistor Rc, and current-detecting resistor Ris determined. The current ratio is maintained to be a predetermined ratio even if the heating current is controlled by an operational amplifierdue to an increase or decrease in airflow rate.

Heating resistor type airflow meter

According to the invention, to correct the curvature characteristic in output due to individual differences between frequency output type hot-wire flow meters, a frequency output type hot-wire flow meter includes a heating resistor disposed in the measurement target fluid and a voltage frequency conversion circuit for converting the air flow rate voltage signal that is proportional to the current flowing through the heating resistor. The voltage frequency conversion circuit converts the air flow rate voltage signal non-linearly to generate the output frequency.

Frequency output type hot-wire flow meter

In a ratiometric output type air flow meter with a heat generating resistor, a ratiometric circuit is provided with a divider circuit which is constituted by invariable resistors and divides an external reference voltage Vref and a multiplier circuit. The invariable resistor is grounded, via the invariable resistor. To one of input terminals of the multiplier circuit an output signal V of an operational amplifier is inputted, to the other input terminal of the multiplier circuit voltage Vex divided by the invariable resistors is inputted, and the output signal Vout of the multiplier circuit is outputted to an engine control unit. A correction use resistor is connected at the junction of the invariable resistor and a current detection resistor, and is grounded via a ground wiring resistance. Thus the voltage drop Vg caused by the ground wiring is compensated by the correction use resistor, and the voltage (Vex&minus;Vg) representing the input to the multiplier circuit is kept constant regardless to the magnitude of a heating current flowing through the heat generating resistor. Thereby, a ratiometric output and heat generating resistor type air flow meter which permits a highly accurate air flow rate measurement is realized without being affected by variation due to the heating current flowing through the heat generating resistor.

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Hitachinaka, Japan

Ryo Ando