Gas flow type angular velocity sensor

A gas flow type angular velocity sensor which is capable of reliably sensing an angular velocity while accurately controlling the working gas flow with temperature compensation by using a pair of heat wires as a gas flow sensor without providing any additional gas flow sensor in the sensor body wherein an angular velocity sensing bridge circuit is provided at its current supply source with a temperature compensating circuit connected in series which temperature compensating circuit is composed of a pair of series or parallel connected resistance elements, one of which is a thermosensitive resistance element disposed in a gas path and the other of which is a reference resistance element disposed outside the gas path.

BACKGROUND OF THE INVENTION 
The field of the present invention relates to a gas flow type angular 
velocity sensor which is capable of detecting a deflection of a gas flow 
in a gas path when an angular velocity acts on the sensor body by sensing 
a change of resistance in each of paired thermosensitive resistance 
elements disposed in the gas path. 
There has been known such a gas flow type angular velocity sensor for 
sensing an angular velocity acting on its body, wherein gas is forced by a 
pump through a nozzle port into a gas path toward a pair of 
thermosensitive resistance elements (heat wires) arranged at the right and 
the left in the gas path and a change of differential resistance in the 
paired heat wires, which is produced when the gas flow is deflected to the 
left or the right by the action of an angular velocity applied to the 
sensor body, is detected by an unbalanced output of an angular velocity 
sensing bridge circuit which includes, in its respective arms, the 
above-mentioned paired thermosensitive resistance elements and paired 
reference resistance elements. 
This type of angular velocity sensor, however, has a drawback that its 
detecting accuracy may vary with a change of the flow rate of gas in the 
gas path because the sensor is designed to determine a deflection of the 
gas flow by sensing a differential change of resistance in the paired 
thermosensitive resistances. 
Recently, there has also been developed a gas rate sensor of the type which 
has a body portion composed of a gas path and a pair of heat wires 
arranged therein and which is manufactured by semiconductor 
micro-machining on the basis of IC technology. Usually, the sensor uses a 
small volume of gas in its gas path and its detecting accuracy, therefore, 
may be greatly affected even by a very small fluctuation of the gas flow. 
Accordingly, Japan Laid-Open Patent Publication No. 5-2026 proposes to 
provide an angular velocity sensor with a flow sensor (additional 
thermosensitive resistance) in a nozzle portion of the sensor body to 
detect a flow rate of gas through the nozzle port and to control the 
operation of a gas injection pump so as to maintain a constant gas flow 
rate. This solution, however, requires the provision of an additional flow 
sensor element in the angular velocity sensor body, which complicates the 
construction of the sensor body. 
The sensitivity of the sensor to a change in the gas flow rate may vary 
with a change in the gas temperature. Therefore, it is also necessary to 
make a temperature compensation together with the control of the gas flow 
rate. 
SUMMARY OF THE INVENTION 
In view of the foregoing, the present invention was made to provide a gas 
flow type angular velocity sensor which is capable of accurately sensing 
an angular velocity acting on its body while controlling the gas flow rate 
by using a pair of heat wires for sensing the angular velocity and the gas 
flow rate (without providing an additional flow sensor) with temperature 
compensation and which is featured by an angular velocity detecting bridge 
circuit provided at its power supply source with a temperature 
compensating circuit that is connected in series therewith and comprises a 
series-connected or parallel connected thermosensitive resistance element 
and reference resistance element.

In the drawings, 1 is a lower semiconductor substrate, 2 is an upper 
semiconductor substrate, 3 is a nozzle hole, 4 is a gas path, 51 and 52 
are paired heat wires (thermosensitive resistance elements), 81 and 82 are 
cold wires (reference resistance elements), 9 is a power supply 
(constant-current source), 10 is an angular velocity detecting bridge 
circuit, 11 is an amplifier, 12 is a temperature compensating circuit, 14 
is a gas flowrate control bridge circuit and 16 is a miniature pump. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will be now described by way of 
example and with reference to the accompanying drawings. 
FIGS. 1 to 3 shows a general construction of a miniature sensor body of a 
gas-flow type angular velocity sensor, which is manufactured by 
micro-machining on semiconductor substrates. The sensor body is 
constructed in such a way that a lower semiconductor substrate 1 having a 
half nozzle port 31 and a half groove 41 etched thereon and an upper 
semiconductor substrate 2 having a similar half nozzle port 31 and a 
similar half groove 41 etched thereon are bonded to each other so as to 
precisely couple the half ports 31 and the half grooves 41 to form a 
nozzle port 3 and a gas path 4 in the assembled body. 
The lower semiconductor substrate 1 has a bridge portion 6 etched thereon 
with a pair of heat wires 51 and 52 formed on the bridge portion 6 by 
patterning. Electrode portions 7 are formed by patterning at both sides of 
the paired heat wires 51 and 52. 
FIG. 4 is illustrative of a configuration of an angular velocity detecting 
circuit of the gas flow type angular velocity sensor. This circuit 
comprises of an angular velocity detecting bridge circuit 10 which 
includes, in its arms, paired heat wires 51 and 52 arranged in the sensor 
body and paired cold wires 81 and 82 serving as reference resistance 
elements disposed outside the sensor body and which is provided with a 
power supply (constant current source) 9 and an amplifier 11 for 
amplifying an output voltage Vr of the bridge circuit 10. 
The gas flow type sensor having a thus constructed sensor body can 
electrically detect a deflection of a laminar flow of inert gas (e.g., 
N.sub.2 or Ar) constantly injected by a miniature pump (not shown) into 
the gas path 4 through the nozzle hole 3 and directed toward the paired 
heat wires 51 and 52 disposed therein. When the gas flow in the gas path 4 
deflects to the left or the right by the action of an angular velocity 
applied to the sensor body, the paired heat wires 51 and 52 change their 
resistances and the bridge circuit 10 produces an unbalanced output 
corresponding to a differential change of thermosensitive outputs of the 
paired heat wires 51 and 52. This output is a detection signal of an 
angular velocity. 
However, the sensor for detecting a deflection of a gas flow in its body by 
sensing a differential change of resistances of the paired heat wires 51 
and 52 may change its sensitivity of angular velocity detection when the 
gas flow rate changes as shown in FIG. 5. 
The sensitivity characteristic of the sensor relative to a change in the 
gas flow rate varies with a change in the working temperature. For 
instance, it varies by a value of .DELTA.S (see FIG. 5) when the 
temperature of the gas flow at acting point "f" changes from 25.degree. C. 
to 80.degree. C. 
The present invention, therefore, provides a temperature compensating heat 
wire 121 which is disposed at a place free from the affect of the gas 
flowing in the gas path 4 in the sensor body as shown in FIG. 2 and which 
is connected to a bridge circuit 10 in series with the power supply 9 
thereof as shown in FIG. 6. Further, a circuit of series-connected cold 
heat wires (reference resistance elements) 131 and 132 and the bridge 
circuit provided with the series heat wire 121 are connected in parallel 
with each other to form a flowrate control circuit 14' of the double 
bridge type wherein the angular velocity detecting bridge circuit 10, the 
temperature compensating heat wire 121 and the cold wires 131 and 132 form 
respective arms of a bridge circuit. An output Vf of the flowrate control 
bridge-circuit 14' is transferred as a gas flowrate detection-signal to a 
gas flowrate control circuit 15 which according to the detection signal 
controls the drive of the miniature pump 16 for injecting the gas into the 
gas path 4 to maintain a constant gas flowrate in the gas path. 
In the shown embodiment, the temperature compensating heat wire 121 is 
formed in the sensor body at a place where it can sense a temperature of 
the sensor without being affected by the gas flowing in the gas path 4, 
for example, on a bridge portion 18 formed across a groove 17 etched on a 
lower semiconductor substrate 1 as shown in FIG. 2. 
In the normal case with the sensor body and the miniature pump placed in a 
container filled with pressure gas to circulate the gas through the sensor 
body by the pump, it is desirable to provide an opening through which the 
groove 17 communicates with the outside of the sensor body in order to 
keep the same pressure in the gas path and the groove 17. 
In the flowrate control bridge-circuit 14', the angular velocity detecting 
bridge-circuit 10 is also used as a flow sensor. Accordingly, it is 
necessary to eliminate the possibility of changing the angular velocity 
detecting accuracy due to the effect of a temperature change, i.e., to 
match the temperature coefficient of a resultant resistance RB of the 
angular velocity detecting bridge-circuit 10 with that of resistance Rth 
of the temperature compensating heat wire 121 so that the sensitivity 
characteristic of the angular velocity sensor versus the output Vf of the 
flowrate control bridge-circuit 14' may not vary with a change of 
temperature and the constant detection accuracy can be maintained at the 
acting point "f" of the gas flow. 
The resultant resistance RB of the angular velocity detecting bridge 
circuit 10 can be calculated according to the following equation: 
EQU RB=Rcw.multidot.Rhw/(Rcw+Rhw) (1) 
where Rhw is the series resistance of heat wires 51 and 52 for detection of 
an angular velocity and Rcw is the series resistance of cold wires 81 and 
82. 
If the series resistance of cold wires 131 and 132 in the flowrate control 
bridge-circuit 14' is expressed as Rref, RB is much less than Rref (i.e. 
RB&lt;&lt;Rref). Consequently, in the current equation I=IB+Iref that flows 
through the flowrate control bridge circuit 14', there is a relation of 
IB&gt;&gt;Iref. 
If the angular velocity detecting bridge-circuit 10 and the temperature 
compensating heat wire 121 are independent from each other in circuitry, 
the temperature coefficient of the heat wire 121 can be controlled by 
changing the current flowing through the heat wire 121 to attain a desired 
temperature. 
In the case of the flowrate control bridge circuit 14' having the 
configuration shown in FIG. 6, the resultant resistance RB of the angular 
velocity detecting bridge-circuit 10 and the resistance Rth of the 
temperature compensating heat wire 121 have different temperature 
coefficients of resistance, which may cause a difference between 
temperature characteristics of the bridge circuit 10 and the heat wire 121 
in relation to the current IB commonly flowing through them. This means 
that matching the temperature coefficients of the bridge circuit 10 and 
the heat wire 121 can be achieved only at a specified current value 
I.sub.B1 at a point where their temperature characteristics meet with each 
other. If a gas flow type angular velocity sensor is designed according to 
a specified design value, it is impossible to control the sensitivity of 
the sensor by changing the current IB to be supplied to the angular 
velocity detecting bridge-circuit 10. 
Accordingly, the present invention also contemplated that the flowrate 
control bridge-circuit 14 is provided with a temperature compensating 
circuit 12 consisting of a cold wire (reference resistance element) 122 
connected in parallel with the temperature compensating heat wire 121 as 
shown in FIG. 9. The cold wire 122 is formed of the same material (e.g., 
platinum) by the same process that the heat wire 121 is formed. 
With the temperature compensating circuit 12 of the heat wire 121 and the 
cold wire 122 connected in parallel with each other, it is possible to set 
any desired temperature coefficient of resistance irrespective of the 
current supplied to the temperature compensating circuit 12 by selectively 
setting a resistance ratio between the heat wire 121 and the cold wire 
122. The setting range runs from a temperature coefficient of resistance 
in the heat wire 121 at a current Ith flowing therein to a temperature 
coefficient of resistance in the cold wire 122. 
The temperature compensating circuit 12 has a configuration similar to the 
angular velocity detecting bridge-circuit 10 and, therefore, its 
temperature characteristic relative to the common current IB is similar to 
that of the angular velocity detecting bridge circuit 10 as shown in FIG. 
10. 
Accordingly, the temperature dependency of the output Vf of the flowrate 
control bridge-circuit 14 is eliminated and, thereby, the gas flow rate 
can be constantly controlled according to the output Vf irrespective of 
the sensor body temperature. The adjustment of the current IB for 
optimizing the sensitivity of the sensor can be achieved free from the 
temperature influence. 
While the preferred embodiment is described it is to be understood that the 
invention is not limited thereto but may be otherwise variously embodied. 
For instance, a temperature compensating circuit 12 may be adopted with 
the thermosensitive resistance elements 121 and 122 connected in series 
with each other, as shown in FIG. 11. The present invention can be applied 
to a sensor of the type which is intended to detect a change of heat 
distribution by the action of momentum, e.g., an acceleration acting on 
the sensor body by using a bridge circuit comprising a heat wire and a 
cold wire together with reference resistance elements. 
It is also possible to construct a flowrate control bridge circuit 14 that 
is not of the double bridge type but rather with the angular velocity 
detecting circuit 10 and the temperature compensating circuit 12 connected 
in series and a reference voltage corresponding to a middle potential is 
applied to the connecting point of the circuits. 
As is apparent from the foregoing description, the gas flow type angular 
velocity sensor according to the present invention is featured by its 
construction that an angular velocity detecting bridge-circuit provided 
with a temperature compensating circuit connected thereto in series with a 
constant current source and a circuit of a pair of reference resistance 
elements connected in series are connected in parallel with each other to 
form a double bridge type flowrate control bridge-circuit which has, in 
its arms, the angular velocity detecting bridge circuit, the temperature 
compensating circuit and the paired reference resistance elements, and 
said temperature compensating circuit is composed of a thermosensitive 
resistance element and a reference resistance connected in parallel 
thereto and disposed outside of the gas path, thereby allowing paired heat 
wires for sensing an angular velocity to work as a flow sensor (without 
using an additional flow sensor in the sensor body) with temperature 
compensation. Accordingly, the flow rate of gas in the gas path can be 
accurately controlled with stable temperature compensation optimally 
stabilize the sensitivity of the angular velocity sensor.