The invention relates to a device for measuring the flow
of a fluid that has a heater as well as first and second temperature sensors arranged
on opposite sides of the heater. The device further comprises an analog-digital-converter
for digitizing the signal from the temperature sensors. This type of flow sensor
is e.g. described in
. It comprises a reference voltage generator for feeding a reference voltage
to the analog-digital-converter.
It is an object of the present invention to provide means
for improving the accuracy of such a device. This object is achieved by the flow
sensor according to the independent claim.
Accordingly, the reference voltage generator, which generates
the reference voltage for the analog-digital-converter, is adapted to decrease the
reference voltage for an increasing flow velocity of the fluid. This is based on
the understanding that the dependence of the measured analog sensing signal on the
flow of the fluid is strongest for a weak flow and becomes increasingly weaker for
stronger flow velocities. By decreasing the reference voltage for higher flow velocities,
the resolution of the converter can be adapted to this behavior by being increased
at such velocities.
Further advantages and advantageous embodiments are illustrated
in more detail in the dependent claims as well as in the following description.
This description makes reference to the attached figures, where
- Fig. 1 is a top view of a possible embodiment of the device,
- Fig. 2 is a schematic sectional view along line II-II of Fig. 1,
- Fig. 3 is a block diagram of a first embodiment of the circuitry of the device,
- Fig. 4 illustrates the sensing signal &Dgr;T, the monitoring signal TP, and
the digitized value of the sensing signal as a function of mass flow with constant
and non-constant Vref input,
- Fig. 5 is a block diagram of a second embodiment of the circuitry of the device.
The device of Figs. 1 and 2 is designed as a semiconductor
chip based on a silicon substrate 1, even though another semiconductor or dielectric
substrate could be used as well. The device could also be built from discrete elements
not mounted to a common substrate.
A recess or opening 2 has been formed in substrate 1 by
etching techniques and is spanned by a membrane 3. A heater 4 extends over membrane
3. In the embodiment of Figs. 1 and 2, heater 4 is formed by three parallel conductors
4a, 4b, 4c, the two outer conductors 4a, 4c being arranged, electrically, in parallel,
while the center conductor 4b (having double cross section) is in series to the
conductors 4a, 4c.
Two sensing thermopiles 6a and 6b, each consisting of a
plurality of thermocouples in series, form first and second temperature sensors.
They are arranged upstream and downstream of heater 4 (the terms "upstream" and
"downstream" being defined in respect to a flow direction F perpendicular to the
longitudinal axis of the heater). Each sensing thermocouple consists of a metal
bar 7 (shown in continuous lines in Fig. 1) formed in a metal layer of the integrated
circuit as well as a polysilicon bar 8 (shown in dotted liens in Fig. 1) formed
in the polysilicon layer of the integrated circuit. The metal and polysilicon bars
7, 8 of each thermocouple are interconnected at a first junction 9 on membrane 3.
The polysilicon and metal bars 8, 7 of two neighboring thermocouples are interconnected
at a second junction 10, which second junction 10 is not located on membrane 3,
but over the bulk of substrate 1.
The basic principle of operation of such a device is e.g.
. A flow along flow direction F causes the distribution of heat from heater
4 to become asymmetric, which creates a difference of temperature at the first junctions
9 of the two thermopiles 6a, 6b. At the same time, the second junctions 10 remain
at substantially constant temperatures due to the heat exchange through the thermally
conducting substrate 1. Hence, the difference of the voltages from the thermopiles
6a, 6b (or any value proportional thereto), which is called the "sensing signal"
in the following, is substantially a measure of the temperature difference &Dgr;T
at the first junctions 9 upstream and downstream of heater 4. This temperature difference
&Dgr;T is a measure of the mass flow of the fluid.
As can be seen in Figs. 1 and 2, two additional monitoring
thermocouples 12a, 12b are provided on the device, each of which is located at the
center of one of the sensing thermopiles 6a, 6b. The monitoring thermocouples 12a,
12b together form a monitoring temperature sensor of the device.
Each monitoring thermocouple 12a, 12b again consists of
a metal bar 7' and a polysilicon bar 8' interconnected at a first junction 9' located
over membrane 3. The first junctions 9' of the monitoring thermocouples 12a, 12b
are located much closer to heater 4 than the first junctions 9 of the sensing thermopiles
6a, 6b and have a temperature substantially corresponding to the temperature in
heater 4. The second ends of the metal bars 7' are e.g. connected to a polysilicon
lead 13 at a second junction 10'. The second junctions 10' are, in the embodiment
of Figs. 1 and 2, located over the bulk of substrate 1.
Each monitoring thermocouple 12a, 12b therefore generates
a voltage substantially equal to the temperature difference between substrate 1
and heater 4. The voltages from the monitoring thermocouples 12a, 12b are added
to generate a monitoring signal TP.
An embodiment of the circuitry of the device is shown in
Fig. 3. It comprises an A/D-converter 16 for digitizing the sensing signal &Dgr;T,
a heater control 17 for controlling the current through heater 4 and a microcontroller
18 for processing the digitized sensing signal and controlling the device. Advantageously,
the circuitry for all the elements 16, 17 and 18 is integrated on substrate 1, but
it can also be formed at least in part by external components.
Basically, A/D-converter 16 can be any type, such as a
successive-approximation ADC, a delta-encoded ADC or a sigma-delta converter. All
such converter designs require a reference voltage Vref and generate a digitized
value of the input signal normalized by Vref. The term "normalized", in this context,
is to be understood such that the output value for a given input voltage depends
linearly on the reciprocal value 1/Vref.
Advantageously, A/D-converter 16 is a sigma-delta converter,
such as it is e.g. disclosed in
DE 101 29 300
. The description of the section "A/D-Wandler" and its associated figure
of that document are incorporated herein by reference.
As can be seen in Fig. 3, the monitoring signal TP is fed
to the reference input of A/D-converter 16 as a reference voltage Vref. The purpose
of this measure is described below.
Heater control 17 of the embodiment of Fig. 3 regulates
the power dissipated by heater 4 to a constant value. Alternatively, it regulates
the current through heater 4 to a constant value.
The dependence of the temperature difference &Dgr;T as
a function of flow v is shown in the upper graph of Fig. 4. For zero flow, &Dgr;T
= 0. With increasing flow, &Dgr;T will start to rise linearly. At larger flows,
however, the temperature of heater 4 (and therefore the monitoring signal TP) will
decrease substantially because of the cooling effect that the passing fluid has
on heater 4. Hence, for increasing flows v, the slope of the curve &Dgr;T(v) will
decrease as shown in the upper graph of Fig. 4.
If Vref of A/D-converter 16 were constant, the digitized
value from A/D converter 16 would follow the curve "Vref = const" of the lower graph
of Fig. 6. As will be understood by those skilled in the art, this will cause a
decrease of measurement resolution for larger mass flows v because the resolution
of the A/D-converter does not change over its range of input voltages.
However, as mentioned above, Vref is not constant, but
rather it is set to the monitoring signal TP. Hence, the resolution of the A/D-converter
will be coarser when the heater temperature is high, while the resolution will be
finer when the heater temperature is low. This leads to a linearization of the digitized
signal at the output of A/D-converter 16 as indicated by the curve "Vref = TP" in
the bottom graph of Fig. 4.
Hence, using the monitoring signal TP as a reference voltage
Vref allows to obtain a more constant digitization resolution over a wider range
of mass flows, which in turn allows an increased accuracy of measurement and/or
it allows to decrease the bit resolution of the A/D-converter.
At the same time, and as mentioned above, since the digitized
value is proportional to the ratio &Dgr;T:TP, variations of the Seeback coefficient
or of other parameters of the used thermocouples tend to be compensated. Such variations
can e.g. be observed when the overall temperature of the device changes, or when
membrane 3 is subjected to stress.
In view of this, it will become apparent that the design
of Fig. 1 has a further advantage: The monitoring thermocouples 12a, 12b are parallel
and close to the corresponding thermocouples of the sensing thermopiles 6a, 6b,
therefore a strain in membrane 2 will affect both the thermocouples 12a, 12b as
well as the thermopiles 6a, 6b in similar manner and will therefore be eliminated
in the output value of A/D-converter 16.
In the embodiment of Fig. 3, the sensing signal TP was
used to generate the reference voltage Vref. In general, any voltage source generating
a reference voltage proportional to a temperature signal indicative of a temperature
of heater 4 can be used. Advantageously, the reference voltage should be proportional
to the temperature difference between heater 4 and substrate 1 - or, since the temperature
of substrate 1 is primarily dominated by the temperature of the fluid, the reference
voltage should be proportional to the temperature difference between heater 4 and
Alternatively, a suitable reference voltage Vref can also
be generated from the sensing signal &Dgr;T itself. Namely, the reference voltage
Vref has to decrease when the sensing signal &Dgr;T increases. This can e.g. be
achieved by using suitable analog circuitry for processing &Dgr;T and for generating
Vref therefrom, but an advantageous implementation uses digital processing as shown
in the embodiment of Fig. 5.
In Fig. 5, the reference voltage Vref is generated by a
digital-analog-converter 20 driven by microcontroller 18.
The device of Fig. 5 is adapted to first measure a raw
digitized value while feeding a fixed, average reference voltage Vref to analog-digital-converter
16. The raw digitized value can then be used to choose a suitable reference voltage,
e.g. using a calibration table in microcontroller 18. Then, a corrected digitized
value is measured using the chosen reference voltage. The suitable reference voltage
is chosen to decrease for increasing values of the raw digitized value in such a
manner that the corrected digitized value is advantageously substantially proportional
to the mass flow of the fluid.
The fixed reference voltage as well as the suitable reference
voltage are both generated under control of microcontroller 18 by means of digital-analog-converter
In the embodiments described so far, membrane 3 forms a
"measuring region" in contrast to the remaining "regular region" of the chip surface
supported by bulk substrate 1. The measuring region has much lower heat conductance
between the components located thereon than the regular region. At least part of
each, the heater 4, the sensing thermopiles 6a, 6b and the monitoring thermocouples,
should be located on the measuring region. As it is known to the person skilled
in the art, membrane 3 can be replaced by various other "membrane structures", where
the term "membrane structure" describes any thin structure arranged over the opening
or recess 2 in the semiconductor chip. The membrane structure can either be formed
by a membrane completely closing the opening or recess in the chip, or by a bridge
or tongue extending over or into the opening or recess.
Instead of using a recess or opening 2, the measuring region
could also be located on top of a layer of material having low thermal conductivity,
such as a nanoporous silicon oxide.
In the embodiment of Fig. 1, the monitoring thermocouples
12a, 12b are arranged upstream and downstream of heater 4. Alternatively, a suitable
temperature sensor can e.g. be mounted in one or both of the gaps between the conductors
4a, 4b, 4c of heater 4.
Even though, in the embodiments shown so far, most temperature
sensors were thermopiles consisting of a plurality of thermocouples in series, it
must be noted that some or all of them may be replaced by single thermocouples (or
vice versa) depending on desired signal voltages and available space.
It must be noted that in the embodiments described above,
each thermocouple used in the various thermopiles or alone consists of two conductors
consisting of a first and a second material, respectively. Advantageously, the same
materials are used for all thermocouples in order to profit from a shared Seeback
coefficient. Advantageously, one material is a polysilicon and the other a metal,
such as aluminum, both being materials conventionally used in integrated circuit
technology. In addition, and again in order to have thermocouples with uniform properties,
all conductors of the thermocouples should be manufactured from a the same two layers
deposited on the substrate, e.g. one being a polysilicon layer and the other a metal
Further, it must be noted that, even though thermocouples
are advantageously used as temperature sensors, other types of temperature sensors,
such as resistive temperature sensors, can be used as well. For example, monitoring
temperature sensor 12a, 12b can be replaced by a resistivity measurement of heater
4 because the resistivity of heater 4 depends on its temperature. In that case,
heater control 17 can be used for generating the reference voltage.
In general, the reference voltage of the present invention
must be generated by a reference voltage generator. Advantageously, the reference
voltage is proportional to the temperature difference between heater 4 and substrate
1. In the embodiment of Fig. 3, the monitoring thermocouples 12a, 12b play the role
of such a reference voltage generator, or any other circuitry generating a signal
indicative of the temperature of heater 4 can be used. In the embodiment of Fig.
5, microcontroller 18 and digital-analog-converter 20 are used as reference voltage
generator, or analog circuitry could play the same role. As mentioned above, heater
control 17 can also be used as reference voltage generator.
With increasing flow velocity, the reference voltage can
be decreased continuously (or quasi-continuously, i.e. in very small steps) or it
can decreased in step-like manner, such as in a small number of larger steps.