Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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B~CKGROUND OF THE DNVENTIoN
Measurement of flow velocities of a medium using a temperature
sensitive electrical resistor which is cooled by the medium which
flcws around it in dependence of the flow velocity thereof, thereby
changing the resistance, are known.
These known devi oe s all comprise three discrete elements:
the sensor wire, an amplifier for amplifying the measuring signal
obtained as a result of the change of the resistivity of the
measuring resistor, and a signal display device. As a result such
devices occupy a lot of space and are not cheap to produce, as each
sensing resistor must be connected to its own amplifier. An arrangement
in which many sensors are used, for instance when measuring flow
velocities on many points on a model of an aeroplane, such as done
at windtunnel tests beoomes very complicated and expensive.
SUMMARY OF THE INVENTICN
The invention provides a flow velocity measuring device of
very small dimensions in which both the sensing resistors and the
amplifier for amplifying the measuring signal are provided by usual
planar silicon technology on a semiconductor chip of about 0.06 inch
x 0.06 mch. The amplifiers are provided in the centre of the chip
in the space delimited by the sensing resistors. The chip is mounted
in such a way that two of the resistors are normal to the flow
and the two other ones parallel to the flow. The temperature decrease
of the resistors which are normal to the flow is somewhat larger
than that of the resistors which are parallel to the flow and as a
result of the positive temperature coefficient of the diffuse~
resistors the bridge becomes unbalanced. m e bridge output signal
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is then a measure of the velocity of the medium flow and when the
cross-section of the conduit in which the medium flows is known,
also of the debit.
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The invention thus provides for a device for measuring
the flow velocity of a medium, comprising two, flat, tempera-
ture sensitive elongated sensing resistors deposited on an in-
sulating semiconductor chip with two respective longitudinal
axes including an angle of about 90, said resistors being con-
nected with two other resistors into a bridge configuration of
which two opposite points are connected to a current source and
of which the tw~ other opposite points are connected to the in-
put of an amplifier made as an integrated circuit of very small
dimensions of which the ccmponents are deposited on the semi-
conductor chip in the space delimited by the two sensing resis-
tors.
With a view of obtaining a compensation for bridge signals
resulting from a change in ambient temperature at the absence
of flow three amplifiers are provided on the chip, the input of
the first one being connected to the opposite points of the bridge,
the inputs of the second one being connected across one of the
resistors and the outputs of the first and the second amplifiers
being connected to the input of the third amplifier with the out-
put constitutes the output of the device.
The device according to the invention has, as a result of itssmall dimensions and low cost a great many applications, not only
-;` in the field of technics, research (metereological and aeronauti-
cal) and the medical field (lung research, blood flow measurements),
; but also in the domestic field. It can be made very cheaply in
great quantities with excellent reproduceability at the same high
standards with which mode~n semiconductors devices are made.
- SURVEY OF THE DRAWrNGS
Figure 1 is schematical plan view of a device according to the
invention;
Figure 2A is a side view of this device, mounted on a conduit
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through which a medium flows;
Figure 2B is a plan view of this device;
Figure lA is a side view of another embodlment, also mounted
on a conduit through which a medium flow is present;
Figure 3B is a plan view of this device.
Figure 1 shcws on a very enlarged scale - about 60 : 1 - a
silicon chip with dimensions of akout 0.06 inch x 0.06 inch, de-
noted by the reference numeral 1, on which four p-type diffused
resistors are provided by the usual planar technolo~y. These
resistors are denoted by the reference numerals 2, 3, 4 and 5. There
are four contact areas, e.g. of aluminum film, denoted by the
reference numerals 6, 7, 8 and 9. The four resistors 2, 3, 4, and 5
are thus connected into a bridge configuration; the contact areas
7 and 9 are connected to a current, denoted by the symbols + and -,
while the bridge signal is taken between the contact areas 6 and 8.
This bridge signal results from the fact that the heat transfer frcm
the resistors to the medium which flows around it (in the direction
of the arrow lC) is greater for the resistors which are normal to the
flow (in this case the resistors 2 and 4) than for the resistors
which are parallel to the flow (the resistors 3 and 5). m e heat
transfer characteristics are generally represented by the formula
Nu = A + Q ~ (a), in which Nu is the Nusselt number for total heat
transfer, A is a structure-dependent constant associated with non-
flow-dependent heat losses, Q is the flow~dependent forced-~onvection
heat transfer and ~ (a) is a function that accounts for the depen-
dence of the heat flow on the angle between the normal to the dif-
fused resistor and the direction of the flow. The principle of the
sensor is thus based on the difference of ~ (~) between the parallel
and the normal resistors. To keep the term A, which is mainly a re-
sult of heat cond w tion within the substrate, as small as possible achip is used with a thickness of 50 /um.
With four perfectly equal resistors 2, 3, 4, and 5 it may be expecbed
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that the sensor is insensitive for changes of the ambienttemperature. ~owever, in m~st cases it will not be possible
to obtain four perfectly equal resistors and for this reason
a signal is derived from one of the resiStQrS which is dependent
upon the ambient temperature and added as a compensating sig-
nal to the measuring signal taken between the points 6 and 8.
In the space 11, delimited by the four resistors 2, 3, 4 and
5 are three integrated amplifiers 12, 13 and 14, each made by
the usual technology and schematically represented by the sym~ol
comm~nly used for an amplifier. The - input of the first ampli-
fier 12 is connected to the contact area 6 while the + input
of the amplifier is connected to the contact terminal 8; the bridge
signal thus appears in amplified form at the output 12a of the
amplifier 12. The - input of the second amplifier 13 is connected
to the contact area 8 while the + input of this amplifier 13 is
connected to the contact area 9; at the output 13a thus appears a
signal which represents the changes of the resistivity of the
resistor 5 with changes in ambient temperature. The outputs 12a
and 13a are connected with the input of the third amplifier 14 and
at the output 15 of this amplifier appears the output signal of the
bridge. The supply voltages for the amplifiers 12, 13 and 14 are
taken from the contact areas 7 and 9.
e figures 2A, 2B, 3A and 3B show two different examples of
embodinents of the mounting cf the chip 1. Figure 2B shows a ring-
shaped supporting element 16 made of suitable material, such as
plastics or a ceramic material which carries four thin contact wires
17, 18, 19 and 20 connected to the respective terminals 21, 22, 23, 24
on the supporting elements. The other ends of the wires 17-20 are
bonded to suitable contact areas on the chip 1, not shown. Figure
2A shcws how this ring is connected to a conduit 25 through which
a medium flows in the direction of the arrow 26.
Figures 3A and 3B show another enbx~lunelt. Here the chip 1 is again
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supported by four wires 27, 28, 29, 30 which at the one end are
connected to tenminals 31, 32, 33, 34 and at the other end bonded
to contact areas on the chip 1, not shcwn. m e terminals 31, 32,
33, 34 are provided in a block-shaped supporting element 35 of
e.g. plastics or a ceramic material with the circular opening 36.
miS supporting element is m~unted on the conduit 37 through
which the medium flows in the direction of the arrow 38.
In a practical embodiment, using a silicon chip of about
0.06 x 0.06 inch with a thickness of 50 /um, with p-type diffuse
resistors with a resistivity of about 300 Ohm/square and a
width - to length ratio of 1:45, the bridge produced a direct
current output signal of 2 /u Volt per meter per second air-
flow; temperature compensation was obtained keeping the Ohm/temperature
ratio cor~stant for temperatures up to 70 centi7rade.
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