Abstrict A temperature regulator, circuit for an air-mass flow meter in
which the temperature regulator circuit includes a bridge circuit
one of whose bridge branches contains a temperature resistor of
substantially higher resistance than a heating resistor in the other
bridge branch. The resistance element of the temperature resistor
normally requires a large amount of space on the sensor for this
purpose. In order to reduce the size of the resistance element,
a voltage divider is provided to reduce the input voltage applied
to the temperature resistor. In order to maintain the bridge balance,
additional circuit elements are provided in the other bridge branch
containing the heating resistor.
Claims What is claimed is:
1. A temperature regulator for an air-mass meter in an air intake
manifold of an internal combustion engine comprising a bridge circuit
having a first branch including a measuring resistor having a temperature-dependent
variable resistance for detecting ambient temperature, and a second
branch including a heating resistor having a temperature-dependent
variable resistance, said first and second branches having central
taps, a bridge null branch connected to said central taps, said
bridge null branch including an operational amplifier having positive
and negative inputs connected to said central taps to correct any
bridge imbalance based on any voltage difference in the bridge null
branch, means connected in front of said measuring resistor for
reducing voltage supplied to said measuring resistor by a factor
k, and further means in the second bridge branch after the heating
resistor for reducing voltage in said second branch by said factor
k to maintain bridge balance, wherein said means for reducing voltage
supplied to said measuring resistor comprises a voltage divider
including first and second resistors and a voltage follower connected
in front of said measuring resistor in series with the first resistor
of the voltage divider, the second resistor of said voltage divider
being connected in parallel to said voltage follower and to said
measuring resistor.
2. The temperature regulator according to claim 1 further comprising
a resistor in said first branch of said bridge circuit in series
with said measuring resistor and in parallel with said second resistor
of said voltage divider.
3. The temperature regulator according to claim 1 comprising a
common carrier to which said measuring resistors and said heating
resistor are connected.
4. A temperature regulator for an air-mass meter in an air intake
manifold of an internal combustion engine comprising a bridge circuit
having a first branch including a measuring resistor having a temperature-dependent
variable resistance for detecting ambient temperature, and a second
branch including a heating resistor having a temperature-dependent
variable resistance, said first and second branches having central
taps, a bridge null branch connected to said central taps, said
bridge null branch including an operational amplifier having positive
and negative inputs connected to said central taps to correct any
bridge imbalance based on any voltage difference in the bridge null
branch, means connected in front of said measuring resistor for
reducing voltage supplied to said measuring resistor by a factor
k, and further means in the second bridge branch after the heating
resistor for reducing voltage in said second branch by said factor
k to maintain bridge balance, wherein said second bridge branch
includes a further resistor in series with said heating resistor,
said further means comprising another resistor in said second branch
in series with said heating resistor to form a further voltage divider,
the central tap of said second branch being between said further
resistor and said another resistor.
Description FIELD OF THE INVENTION
The invention relates to a temperature regulator, particularly
for an air-mass flow meter.
BACKGROUND AND PRIOR ART
Air-mass flow meters are used particularly for determining flow
of inlet air to an internal combustion engine. Heating element anemometers,
also denoted thermal air-mass flow meters, are conventionally used
for the design of air-mass flow meters. Two sensors or detectors
are provided, one of which detects the temperature of the aspirated
air and the other of which is heated to a specific temperature.
The detectors are disposed in different branches of a bridge circuit.
The detector that is heated and serves as the measurement sensor
is cooled by the air flow depending on its velocity and temperature.
The additional energy input required for bridge balance is thus
a measure of the air mass flow that has passed through the detector.
When there are large deviations from a reference temperature, however,
measurement errors arise, due to the change in thermal conductivity
of the air with respect to temperature.
In order to compensate for this measurement error, it is known,
for example, to use a third sensor, as disclosed in DE 37 22 385
A1. Thereby, the temperature-dependent measurement error is corrected
by suitable setting of the basic resistance value and its temperature
coefficient.
A temperature regulation circuit for achieving a constant temperature
of a heating resistance in a thermal air-mass flow meter is disclosed
in DE 41 30 513 C2. Also, a high-resistance measuring resistor detecting
the ambient temperature and a heating resistor are arranged in two
different branches of a bridge circuit. By a suitable selection
of resistance ratios of one resistor to the other, the heating resistor
heats up in order to balance the bridge. The bridge branch with
the temperature resistor essentially has a higher resistance than
the bridge branch with the heating resistor, so that the intrinsic
heat of the temperature resistor is kept negligibly small in order
to exclude thermal measuring errors. It is a disadvantage that the
high-resistance temperature resistor used for this purpose has a
large space requirement for the sensor.
SUMMARY OF THE INVENTION
An object of the invention is to provide a temperature regulator,
which is reduced in its size while maintaining the bridge conditions.
The invention is based on the concept that in spite of the condition
that one bridge branch is configured with higher resistance compared
to the other branch, in order to reduce the intrinsic resistance
values, the size can also be kept small.
This is achieved by creating a voltage reduction in both bridge
branches by additional means. With a smaller supply voltage, the
resistance value of the temperature sensor can consequently be reduced,
whereby the dimensions of the temperature sensor are also reduced.
The same reduction in voltage is produced in the other bridge branch
containing the heating resistor in order to keep the bridge in equilibrium.
Subsequently, a post-regulation of the heating resistance is produced
in a conventional manner by an operational amplifier upon an imbalance
in the bridge caused by an increased temperature in the resistance
of the temperature sensor. For this purpose, the resistance of the
temperature sensor and the heating resistance have the same type
of resistance values that change in a temperature-dependent manner.
The central taps of the two bridge branches that form the null branch
of the bridge are joined to the positive and negative inputs of
the operational amplifier.
By the selection of smaller resistance values and the thus associated
smaller dimensions of the resistor of the temperature sensor and
the heating resistor, these components may also be connected to
a common carrier, so that the flow direction can also be determined
in a simple way.
BRIEF DESCRIPTION OF THE DRAWING
The sole FIGURE of the drawing shows a portion of a circuit of
a temperature regulator. Only the essential components necessary
for understanding the invention are illustrated.
DETAILED DESCRIPTION
In the drawing, there is shown a temperature regulator 1 in the
form of a bridge circuit. In one branch of the bridge circuit is
a temperature sensor constituted as a high-resistance temperature-dependent,
variable measuring resistor R1 for detecting only the ambient temperature,
in series with a resistor R2. In the second branch of the bridge,
there is a temperature-dependent, variable heating resistor R3
in series with a resistor R5. Resistors R1 and R3 have positive
temperature coefficients and a temperature dependence that is equalized
as much as possible.
In addition to this conventional arrangement, temperature regulator
1 according to the invention has means 13 which reduces the input
voltage to temperature resistor R1. Means 13 comprises a voltage
divider formed by resistors R6 and R7 and a voltage follower 15.
Voltage follower 15 is connected in front of measuring resistor
R1 and in series with resistor R6 whereas the other resistor R7
is incorporated in the bridge circuit in parallel with voltage follower
15 and resistors R1 R2. A resistor R4 is connected in the second
bridge branch in series between resistors R3 and R5 to form a further
means 14 for voltage reduction. The central taps between resistors
R1 R2 on the one hand, and R4 R5 on the other hand belong to the
bridge null branch and are respectively connected to negative input
11 and positive input 12 of an operational amplifier 10. The bridge
circuit is supplied by a supply voltage source V.sub.cc and is connected
on one side to electrical ground, so that resistors R2 R7 and R5
have a common ground reference potential.
Supply voltage U.sub.s of the first bridge branch, i.e., the measuring
branch with the temperature-measuring resistor R1 is reduced by
resistors R6 and R7 acting as a voltage divider, whereby the resistance
ratio of the two resistors R6 R7 determines a factor k by which
supply voltage U.sub.s is reduced. This factor k is expressed by
the relationship k=R7/(R6+R7) and is always less than 1. Supply
voltage V.sub.cc in the first bridge branch is reduced by this factor
k and supplied to voltage follower 15. Voltage follower 15 also
referred to as an impedance transformer, is a negative feedback
d.c. amplifier, whose output voltage is equal to its input voltage
and which serves for decoupling temperature measuring resistor R1
from voltage divider R6 R7 in order to avoid a falsification of
the voltage divider ratio. In order to maintain bridge equilibrium,
the ratio of resistors R4 and R5 is likewise adjusted so that the
output voltage of the other bridge branch is multiplied by the same
factor k. and thus in the normal state, there is no difference between
the bridge taps.
A post-regulation of temperature occurs in the conventional way,
if a disruption causes decrease or increase of the temperature of
heating resistor R3 and thus causing its resistance value also
to change. This temperature regulation is produced via operational
amplifier 10. The post-regulation is produced differently depending
on the circuit and can be connected individually. Thus, a regulating
transistor (not shown) can be incorporated between supply voltage
source V.sub.cc and the bridge circuit, and operational amplifier
10 acts on this transistor in such a way that it opens it further
or closes it further. However, it is also possible to apply the
output of operational amplifier 10 directly to the bridge connection
in front of resistors R6 and R7. In this way, regulation is always
produced, so that the temperature differences between R3 and R1
is kept constant.
For example, an air-mass flow meter in the air intake manifold
of an internal combustion engine can be operated with the temperature
regulator 1 such that the heating element of the air-mass flow
meter corresponds to heating resistor R3 of the circuit. If its
elevated temperature is kept constant, the magnitude of the air
flow can be evaluated from the flow of heat or the parameter associated
therewith.
Thus, temperature resistor sensor R1 assumes a resistance value,
which characterizes the temperature of the flowing medium. Depending
on the mass of the passing air, the temperature resistor sensor
R1 is cooled to a greater or lesser extent, which leads to a detuning
or an imbalance of the bridge circuit. The current necessary for
correcting this imbalance is then a measurement of the air mass
flowing past temperature resistor R1.
Temperature regulator 1 can be modified in many ways. For example,
resistors R1 and R3 can alternatively have negative temperature
coefficients, but then must be reversed at inputs 11 12.
Although the invention is disclosed with reference to a particular
embodiment thereof, it will become apparent to those skilled in
the art that numerous modifications and variations can be made which
will fall within the scope and spirit of the invention as defined
by the attached claims. |