Abstrict A quantity of flow meter, includes two electrical conductors which
are traversed by current and are physically traversed by a flowing
medium to be measured. The increase of the current required for
maintenance of the temperature difference between the two conductors
is evaluated as a measurement of the quantity of flow. The meter
is particularly versatile and can be employed for a wide voltage
range and includes resistors as the electrical conductors which
are thin resistor layers applied upon a thin carrier.
Claims We claim:
1. In a flow meter of the type in which a flowing medium is directed
past two electrical conduction elements which have a current passing
therethrough, one of the conduction elements constituting a measuring
resistor which is heated by the current and the other constituting
a comparator resistor which is not heated by the current to any
practical extent, and in which the measuring and comparator resistors
are for connection in a bridge which has a tap and which is connected
to a regulator circuit for controlling the current through the bridge
in response to the voltage arising at the tap, the change in bridge
current representing the quantity of the flowing medium, the improvement
therein comprising:
a carrier;
a first thin layer resistor carried on said carrier as said measuring
resistor; and
a second thin layer resistor carried on said carrier as said comparator
resistor and having a resistance value which is at least ten times
that of said measuring resistor.
2. The improvement of claim 1 and further comprising:
a reinforcement substrate mounting said carrier; and
wherein said reinforcement substrate comprises, at least in the
area of said resistors, openings and said carrier is bonded to said
substrate at areas outside of said openings.
3. The improvement of claim 1 and further comprising:
a heat sink between said measuring and comparator resistors.
4. The improvement of claim 3 wherein:
said heat sink comprises metal.
5. The improvement of claim 3 wherein:
said heat sink comprises a plastic material.
6. In a flow meter of the type in which a flowing medium is directed
past two electrical conduction elements which have a current passing
therethrough, one of the conduction elements constituting a measuring
resistor which is heated by the current and the other constituting
a comparator resistor which is not heated by the current to any
practical extent, and in which the measuring and comparator resistors
are for connection in a bridge which has a tap and which is connected
to a regulator circuit for controlling the current through the bridge
in response to the voltage arising at the tap, the change in bridge
current representing the quantity of the flowing medium, the improvement
therein comprising:
a carrier;
a first thin layer resistor carried on said carrier as said measuring
resistor and comprising a metal layer having a width to thickness
ratio by at least 10:1; and
a second thin layer resistor carried on said carrier as said comparator
resistor.
7. In a flow meter of the type in which a flowing medium is directed
past two electrical conduction elements which have a current passing
therethrough, one of the conduction elements constituting a measuring
resistor which is heated by the current and the other constituting
a comparator resistor which is not heated by the current to any
practical extent, and in which the measuring and comparator resistors
are for connection in a bridge which has a tap and which is connected
to a regulator circuit for controlling the current through the bridge
in response to the voltage arising at the tap, the change in bridge
current representing the quantity of the flowing medium, the improvement
therein comprising:
a carrier;
a first thin layer resistor carried on said carrier as said measuring
resistor;
a second thin layer resistor carried on said carrier as said comparator
resistor; and
a temperature compensation resistor carried on said carrier.
8. The improvement of claim 7 and further comprising:
a pair of heat sinks on said carrier isolating said resistors from
mutual thermal influences.
9. In a flow meter of the type in which a flowing medium is directed
past two electrical conduction elements which have a current passing
therethrough, one of the conduction elements constituting a measuring
resistor which is heated by the current and the other constituting
a comparator resistor which is not heated by the current to any
practical extent, and in which the measuring and comparator resistors
are for connection in a bridge which has a tap and which is connected
to a regulator circuit for controlling the current through the bridge
in response to the voltage arising at the tap, the change in bridge
current representing the quantity of the flowing medium, the improvement
therein comprising:
a carrier in the form of a thin plastic layer dimensioned so that
the same will flutter in the flowing medium;
a first thin layer resistor carried on said carrier as said measuring
resistor; and
a second thin layer resistor carried on said carrier as said comparator
resistor.
10. In a flow meter of the type in which a flowing medium is directed
past two electrical conduction elements which have a current passing
therethrough, one of the conduction elements constituting a measuring
resistor which is heated by the current and the other constituting
a comparator resistor which is not heated by the current to any
practical extent, and in which the measuring and comparator resistors
are for connection in a bridge which has a tap and which is connected
to a regulator circuit for controlling the current through the bridge
in response to the voltage arising at the tap, the change in bridge
current representing the quantity of the flowing medium, the improvement
therein comprising:
a frame including a reinforcement substrate having openings therethrough;
a carrier on said reinforcement substrate covering the openings;
a first thin layer resistor carried on said carrier spanning one
of the openings and constituting said measuring resistor; and
a second thin layer resistor carried on said carrier spanning another
of the openings and constituting said comparator resistor.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a quantity of flow meter, in the
case of which a flowing medium is directed past two current carrying
conductors which have a temperature-dependent resistance, in the
case of which one electrical conductor is heated by the current
and serves as a measuring resistor and the second conductor serves
as a comparator resistor and is not heated by the current. More
specifically, the invention relates to a quantity of flow meter
of such type in which further resistors are provided which, together
with the measuring resistor and the comparator resistor, form a
resistance measuring bridge, and the provision of devices for electronically
controlling bridge current in dependence upon the voltage at a bridge
tap, the devices regulating the voltage at the bridge tap to zero,
the change of bridge current being evaluated as a measurement for
the quantity of the flowing medium.
2. Description of the Prior Art
A flow meter of the type set forth above is known from "Bosch
Technische Berichte 5 (1975) 1" in which the comparator resistor
and the measuring resistor are formed by clamped thin wires. For
the comparator resistor, a significantly thinner wire is necessary
than for the measuring resistor, so that a comparator resistor can
be produced with such a high resistance value that it does not heat
to an interfering degree by means of the current flowing therethrough,
in contrast to the measuring resistor. The sizes of the wires are
set at lower limits because of the necessary mechanical strength
of the wires. Therefore, as a result of the small dimensions of
the flow meter which are required, the resistance cannot be selected
to be so high that a sufficient accuracy of measurement is attained.
The dimensions required, for example in motor vehicle construction,
of in any case but few centimeters, produce such low resistance
values that the bridge can be operated only with voltages in the
order of magnitude of 1 V. In the case of such small voltages, for
example, in flowing liquids, galvanic voltages between different
parts of the circuit, which are to be avoided, come into play. Such
voltages cause errors in the measured result in a manner which is
not controllable. In flowing gases, for example air, as a result
of the low resistance values, in the case of a bridge voltage of
1 V, at the measuring resistance, excess temperatures of about 200
K. arise, which overload the measuring resistance for a long time
and are not permissible for many flowing media. In this respect,
an excess temperature up to 40 K. is not dangerous, and such a temperature
can only be set in the case of resistance wires with bridge voltages
significantly below 1 V, whereby the bridges operate inaccurately.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a quantity of
flow meter which can be operated with voltages which exclude an
interfering influence of galvanic voltages of the circuit parts,
and which makes possible a sufficient measurement accuracy in the
case of an excess temperature which lies markedly under 200.degree.
C.
The above object is achieved, according to the present invention,
in that the measuring resistor and the comparator resistor are designed
as thin resistor layers which are applied upon a thin substrate.
Quantity of flow meters of this type are less sensitive to dirt
accumulation than those having wires, since the measuring resistor
and the comparator resistor display a comparatively large surface
and a specific dirt accumulation distributes itself upon a larger
area, whereby the measurement is influenced to a lesser degree.
Because of the large ratio of surface to thickness of the resistor
layers, their response time is very short. In addition, a very low
excess temperature suffices in order to obtain a current change,
which makes possible the measurement of the velocity of flow with
the necessary precision. Since the strength of the resistors is
not determined by means of the resistor material, as in the case
of a construction using resistor wires according to the prior art,
resistance values which are higher by a multiple can be housed upon
a small available space, so that with the voltages which are present,
in motor vehicles, for example 12 V, the quantities flowing can
be measured with an accuracy of, for example, .+-.2%.
The excess temperature at which the precision resistor heats up
with respect to the surrounding temperature lies advantageously
at 40 K. Thereby, in continuous operation, a thermal overload of
the flow meter is avoided and it makes possible the employment of
the flow meter for measuring flowing liquids which have relatively
low boiling points. The quantity of flow meter is particularly suitable
for being used in the motor vehicle sector. It makes possible the
continuous measurement of the air and fuel quantities which are
necessary for an electronically controlled fuel/air dosage, which
quantities are sucked in by internal combustion engines. It provides
independent measured values at least in the prescribed temperature
range of -35.degree. C.-+150.degree. C. from the temperature of
the flowing medium. Its response time, determined within the flow
changes up to the e.sup.th part, lies by two to three powers of
10 below the limit of the allowable response time of 5 ms.
It is advantageous for a flow measurement in the flow center that
the two resistors be arranged one after the other. If a larger flow
cross-section is to be encompassed by the measurement, then the
precision resistor and the comparator resistor are arranged advantageously
next to one another and transversely to the flow direction. For
contacting of the resistor paths, advantageously conductive paths
are partially vaporized. For this, in particular, copper is suitable,
to which connection wires, or respectively, leads, are soldered
on.
The measuring resistor advantageously possesses a small resistance
value, whereas the comparator resistor has a large resistance value.
In the case of connection of a common voltage source, there arises
in the comparator resistor a dwindlingly low heating, so that the
resistance value does not change in dependence upon the quantity
of flow.
A high change in resistance in the heat abstraction by means of
the flow is attained by the formation of the measuring resistor
out of nickel. The resistance value of the measuring resistor is
advantageously smaller by at least the factor 10 than that of the
comparator resistor. The comparator resistor is advantageously formed
as a meander-shaped metal layer, particularly if the longitudinal
expanse of the comparator resistor is not to be larger than that
of the measuring resistor. Thereby, the comparator resistor and
the measuring resistor advantageously consists of the same material.
Therefore, the temperature influence is compensated in a simple
manner.
Because of the short response time of the flow meter constructed
in accordance with the present invention, it is possible and practical
to coat the resistor layers with a thin lacquer layer out of an
antiadhesive substance, preferably, out of a silicon resin, for
example, 0.5 .mu.m-1 .mu.m thick, for the avoidance of deposits
of dirt. A protective layer of this sort increases the response
time insignificantly. For example, in the case of a 25 nm thick
nickel resistor layer, the response time changes from 10.sup.-6
s to 5.times.10.sup.-6 s, and the response time thus does not come
into the order of magnitude of the permissible limit.
The temperature of the flowing medium is essentially eliminated
in the case of the proposed flow meter by means of the bridge principle;
however, it enters in, though indirectly, to the excess temperature.
For the exclusion of this influence, the current which is supplied
to the bridge must be varied with the temperature. If, in addition,
the temperature of the flowing medium is to be determined directly,
advantageously a temperature-dependent resistor is used. This resistor
is loaded with a constant low current which does not raise the resistance,
so that the voltage drop at this resistor is a measurement for the
temperature. This additional resistor is advantageously formed from
a vaporized metal layer, from which also measuring resistor and
the comparator resistor are formed, and it lies in its own circuit,
independent of the bridge current.
A rational manufacturing in series (quantity production) of the
resistors is provided by means of an embodiment in which the measuring
resistor and the comparator resistor are applied upon a thin carrier
and are connected with one another in one piece and in which the
thin carrier is glued onto a reinforcement substrate. The resistors
can be manufactured by means of a known foil evaporation method
in the moving tape technique, as well as the photo etching or screen
printing techniques and can be glued together with the reinforcement
substrate only shortly before completion. The ratio of width to
thickness of the resistor layer of the measuring resistor is advantageously
at least 10:1 in order to attain a short response time.
The reinforcement substrate is perforate in the region of the resistor
layers, whereby the remaining parts of the reinforcement substrate
forms a frame and the thin carrier is clamped on the frame. Thereby,
the direct basis of the resistors displays a comparatively low thermal
capacity, so that the excess temperatures which are required for
the flow measurement in the measuring resistor can be produced with
relatively narrow (for example, 0.5 mm wide) paths of the same,
and correspondingly low currents pass therethrough, for example
.ltoreq.0.5 A. The necessary mechanical strength of the flow meter
is guaranteed completely by means of this window-like form of the
reinforcement substrate.
In order to avoid a mutual influencing of the measuring resistor
and the comparator resistor, the carrier is connected with the thin
resistor layer between the measuring resistor and the comparator
resistor with a central rib as a heat sink in a materially bonded
manner. The central rib can be a metal layer which is directy vapor
deposited onto the resistor layer, the metal layer simultaneously
being employed as a contact surface. It can also, where applicable,
additionally be formed by a rib of the reinforcement substrate or
by a plastic cross piece which is glued on.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the invention, its organization,
construction and operation will be best understood from the following
detailed description, taken in conjunction with the accompanying
drawings, on which:
FIG. 1 is a schematic circuit diagram of a measuring bridge constructed
in accordance with the present invention;
FIG. 2 is a plan view of a measuring resistor and a comparator
resistor constructed in accordance with the present invention;
FIG. 3 is a plan view of a measuring resistor and a comparator
resistor constructed in accordance with the invention; and
FIG. 4 is a plan view of a measuring resistor, a comparator resistor
and a temperature compensating resistor constructed in accordance
with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to FIG. 1 a bridge is illustrated which comprises
a plurality of resistors R.sub.1 R.sub.2 R.sub.1 ' and R.sub.2
'. A differential amplifier D has its inputs connected to taps of
the bridge and causes a bridge balance by way of a regulator circuit
RS by means of change of the bridge current I by the differential
current .DELTA.I. A measuring device M illustrates the differential
current .DELTA.I or, respectively, the quantity of flow of the medium
corresponding to the differential current .DELTA.I.
In FIGS. 2-4 the resistor paths are broadly hatched and the contact
surfaces are narrowly hatched for the purpose of clarity and not
to indicate sectioning.
Referring to FIG. 2 a measuring resistor R1 and a comparator R2
are arranged next to one another on a thin carrier 1 which preferably
comprises a plastic layer. The carrier 1 is connected in a materially
bonding manner with a reinforcement substrate. The reinforcement
substrate 2 includes recesses 3 in the region of the resistors R1
and R2. The recesses provide for very short response times of the
quantity of the flow measuring device. The current feed proceeds
from a contact surface 5 a conductive path located upon a central
rib 4 and a contact surface 7. The central rib 4 prevents a mutual
thermal influencing of the resistors R1 and R2. The central rib
4 comprises a rib in the reinforcement substrate 2 the part of
the carrier 1 which lies thereover and the conductive path which
is applied on the carrier 1 for the current feed. This embodiment
can be designed relatively small and is particularly suitable for
the arrangement transverse to the direction of flow of the medium.
It thereby encompasses a large part of the flow cross-section. All
connections 5 and 6 can be tapped on one side.
Referring to FIG. 3 the resistors R1 and R2 are arranged one after
the other. The central rib 4 again prevents a mutual thermal influencing
of the resistors R1 and R2. It simultaneously represents the contact
surface 7 which connects the two resistors with one another. The
bridge tap is provided by the contact surfaces 6.
Referring to FIG. 4 the resistors R1 and R2 are constructed in
the same manner as in FIG. 2. In FIG. 4 however, the device is
supplemented by a resistor R3 which is provided for the purpose
of temperature compensation. The resistor R3 also lies over a recess
3 in the reinforcement substrate 2. The connections 8 of the resistor
R3 are separated galvanically from the connections 5 and 6. The
direction of flow of the flowing medium preferably corresponds to
the direction of the arrows. Central ribs 4 and 9 prevent an interfering
mutual thermal influencing of the resistors R1 R2 and R3.
Although we have described our invention by reference to particular
illustrative embodiments thereof, many changes and modifications
of the invention may become apparent to those skilled in the art
without departing from the spirit and scope of the invention. We
therefore intend to include within the patent warranted hereon all
such changes and modifications as may reasonably and properly be
included within the scope of our contribution to the art. |