JPH073354B2 - Zero correction circuit - Google Patents

Description

The present invention relates to a zero point correction circuit in a flow measuring device using a microbridge.

[Conventional technology]

FIG. 6 is a perspective view showing an example of a flow rate measuring sensor chip to which the circuit of the present invention is applied. Such a chip structure and a manufacturing method are known, for example, from Japanese Patent Application Laid-Open No. 61-88532, and the outline thereof is a heater resistance RH on the surface of the bridge 15 formed in the central portion of the upper surface of the semiconductor chip 10.
Temperature sensors RU and RD are provided across this.
Place such a sensor chip in the flow and
The RU and RD should be positioned so that they are above and below the flow (eg under the flow in the direction of the arrow) and the heater resistance
When the RH is energized, the upstream side sensor RU is cooled by the flow and the downstream side sensor RD is heated by the heater heat, so that a temperature difference occurs. This temperature difference is taken as a voltage value and taken as a flow rate output.

In the zero point correction circuit in the flow rate measuring device using the conventional microbridge, the offset voltage is manually set to zero. However, in such a circuit, when the microbridge is out of balance due to changes in ambient temperature,
Alternatively, there is a problem in that the fluctuation of the zero point that occurs when an offset voltage occurs in the amplifier cannot be prevented.

A zero-point correction circuit that eliminates such problems is a practical application.
-181946. This circuit is shown in FIG. In the figure, 1 is a heater circuit, MB shown by a dotted line
Is a microbridge, R1 and R2 are sensor load resistors, RH is a heater resistor, RU and RD are temperature sensitive resistors, U1 to U5 are amplifiers, VR1 is a variable resistor, S1 to S3 are switches, C1 is a capacitor, and R3 and R4 are
Is resistance.

The circuit of FIG. 5 has two operation modes, a zero compensation mode and a measurement mode. In the zero compensation mode, switch S1 to the a contact side, switch S2 on, and switch S2 on.
S3 is off and the offset voltage V _os appearing at the amplifier U3 inverted by the amplifier U4, stores the output voltage -V _os to the capacitor C1.

In the measurement mode, switch S1 to the b contact side,
The switch S2 is turned off and the switch S3 is turned on. As a result, the voltage −V _os stored in the capacitor C1 is added to the amplifier U3 through the amplifier U5 and the switch S1, so that the amplifier U3
In the output signal X of, _Vos will be canceled.

As described above, in the compensation mode, the heater is turned off and the reference state of zero flow rate is forcibly generated even if the fluid flows along the microbridge. By periodically inserting such a compensation mode manually or automatically during measurement, zero point compensation can be performed without stopping the flow.

[Problems to be Solved by the Invention]

However, due to the thermal imbalance between the two temperature-sensitive resistors RU and RD of the microbridge and the difference in the temperature coefficient of resistance, there are two temperature-sensitive resistors R when the heater is off and when the heater is on.
Since the U and RD balance states are different, the offset value obtained when the heater is off may not be appropriate as the correction value when the heater is on. In particular, this deviation becomes a problem when the measured flow rate is small and the amplifier gain is high.

The present invention has been made in view of the above circumstances, and an object thereof is to make the value of the output signal of the amplifier indicate zero flow rate when the actual flow rate is zero.

[Means for Solving the Problems]

In order to solve such a problem, the present invention relates to a zero point correction circuit having an automatic zero point correction function, and a correction means for correcting a zero point shift between a reference state forcibly generated and an actual measurement state. And the error of the zero point in the measurement state is removed by this correction means.

[Action]

In the zero point correction circuit according to the present invention, when the flow rate is zero, the value of the output signal of the amplifier shows zero flow rate.

〔Example〕

FIG. 1 is a circuit diagram showing an embodiment of a zero point correction circuit according to the present invention. The difference from FIG. 5 is that the voltage VA is applied to the a contact of the switch S1 by the variable resistor VR2 as the correction means.
Is supplied. The switch S1 falls to the a-contact side when the heater is off and to the b-contact side when the heater is on.
By applying the voltage VA generated by the variable resistor VR2 to the a contact when the heater is off, it is possible to cancel the difference in offset caused by the heater off and the heater on. Adjust the variable resistor VR2 at the time of shipment with the heater turned on and the flow rate set to zero. The change in offset due to aging is performed by turning off the heater while the fluid is flowing, as in the conventional case.

FIG. 2 is a circuit diagram showing a second embodiment of the present invention, in which an amplifier U6 is added as compared with FIG. Variable resistance VR
By generating a voltage of -VA by 2 and moving the bias of the entire amplifier, a function equivalent to that of the first embodiment is obtained.

FIG. 3 is a circuit diagram showing a third embodiment of the present invention and shows an example in which an A / D converter 2 and an arithmetic unit 3 are used instead of the amplifiers U4 and U5. In FIG. 3, the switch S4 is tilted to the a contact side when the heater is off and to the b contact side when the heater is on. The A / D converter 2 fetches data when the heater is off and when it is on, and the arithmetic unit 3 calculates the difference between when the heater is off and when it is on. However, the -VA generated by the variable resistor VR2 is added to the data when the heater is on. In the calculation result, the offset difference caused by turning the heater off and on is canceled. X is the data when the heater is off and when it is on, and Y
Is a flow rate value calculated by the arithmetic unit 3.

FIG. 4 is a circuit diagram showing, as a fourth embodiment, an example in which the zero-point correction value is not set as the variable resistance setting value shown in FIG. 3 but is written in the PROM 4 as a digital value. In the measurement, the zero point correction value from the PROM 4 is given to the arithmetic unit 3, and this zero point correction value is added to the measurement value.

〔The invention's effect〕

As described above, according to the present invention, the offset voltage when the heater is turned on can be set to zero by correcting the deviation between the reference state and the actual measurement state, so that the output of the amplifier when the actual flow rate is zero. This has the effect of allowing the value of the signal to indicate zero flow.

[Brief description of drawings]

FIGS. 1 to 4 show the zero-point correction circuit according to the present invention.
FIG. 5 is a circuit diagram showing a fourth embodiment, FIG. 5 is a circuit diagram showing a conventional zero point correction circuit, and FIG. 6 is a perspective view showing an example of a flow rate measuring sensor chip to which the circuit of the present invention is applied. 1 ... Heater circuit, MB ... Micro bridge, R1, R2 ... Sensor load resistance, RH ... Heater resistance, RU, RD ... Temperature sensitive resistance, U1
~ U5 ... Amplifier, VR1, VR2 ... Variable resistance, S1 ~ S3 ... Switch, C1 ... Capacitor, R3, R4 ... Resistor.

Claims (1)

[Claims] 1. A zero point correction circuit for correcting the zero point of a flow meter using temperature sensors provided on both sides of a heater as an element of a bridge circuit, wherein the flow meter output signal is present when the flow rate is zero when the heater is energized. Offset compensation means (VR2) for compensating for the offset of the zero point correction, storage means (U4, U5) for storing the compensated flowmeter output signal level, and storage contents of the storage means at the time of flow rate measurement. A zero-point correction circuit comprising a switching circuit (S1) for sending in place of the offset compensating means.