Abstrict A current controlled electronic circuit for a hot wire air flow
meter having a symmetrical bridge circuit to measure the mass of
the airflow. The present current bridge arrangement requires very
few adjustments to select a few resistors for obtaining a required
temperature differential and compensation for variations in the
temperature coefficient of resistance of both the heated and cold
resistors. This is accomplished by placing of a voltage divider
in parallel with the sensing leg and a first amplifier responding
to the voltage divider and the sensing let to control the current
in the compensation leg. A second amplifier responds to the compensation
leg for controlling the power to the sensing leg.
Claims I claim:
1. A current controlled electronic circuit comprising:
power driver means, adapted to be connected to a voltage source
for supplying current response to a control signal; and
means for maintaining a desired temperature offset between a first
temperature variable resistor and a second temperature variable
resistor comprising:
a current bridge adapted to be positioned in an air-flow having
a sensing leg including said first temperature variable resistor
and a first offset resistor connected in electrical series and a
compensation leg including said second temperature variable resistor
and a second offset resistor connected in electrical series;
current stabilizing means connected to said sensing leg and compensation
leg for causing the current flows in such leg to be a specified
ratio, including a first amplifier having its positive input connected
to a first resistor which is connected in series with said sensing
leg and a second resistor, the output of said first amplifier is
connected to a first transistor which completes a series connection
between said compensation leg and said second resistor, said first
and second resistors are chosen to generate said specified ratio;
a first voltage divider connected across said sensing leg, said
divider comprising third and fourth resistors having approximately
the same ratio as said first and second resistors;
a second amplifier having a positive input connected to a junction
between said third and fourth resistors and its negative input connected
to sense the voltage across said compensation leg and an output
of said second amplifier communicated to and used to regulate said
power driver means.
2. The circuit as defined in claim 1 including an output amplifier
communicated to sense the voltage drop across the sensing leg to
generate a signal indicative of the air flow.
3. The circuit as defined in claim 2 where the power driver means
is a Power transistor and wherein the output of the second amplifier
is connected to the base of the power transistor.
4. The circuit as defined in claim 3 wherein a buffer amplifier
is connected between the output of the second amplifier and the
base of the power transistor.
5. The circuit as defined in claim 4 wherein the value of the first
temperature variable resistor is given by: ##EQU2##
when G is a gain function to compensate for differences in the
TCR's of the resistors R.sub.h and R.sub.c and
OS is a resistance equivalent to the desired temperature offset.
Description BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to a symmetrical bridge circuit useful
in a mass flow air sensor to measure the mass of airflow into an
internal combustion engine.
The basic measurements in fuel injection systems are the measurements
for determining the amount of fuel to be supplied to the engine.
Air/fuel ratios are satisfied by measuring the amount of air intake
to the engine and then supplying the proper amount of fuel under
control of scheduling tables located in the electronic control unit
(ECU).
The most common method to determine the amount of fuel to be injected
is to measure the manifold pressure and engine speed and from these
measurements determine the amount of fuel. Hot wire anemometers
and swirl meters are examples of devices for measuring the amount
of air flowing into the engine and with this measurement, the amount
of fuel is calculated.
In many devices, a bridge circuit is used in the measuring circuitry.
The bridge consists of a heated leg responsive to air flow and a
unheated leg responsive to air temperature. The unheated leg is
used to compensate the anemometer for changes in air temperature.
The unheated leg must not be heated by the circuit. In some, a low
resistance is used in the heated leg and a high resistance element
is used in the unheated leg. In these situations, both elements
are operated at the same voltage requiring the matching of the temperature
coefficients of the resistance material with the different values
of resistance. It is desirable to use the same type element in both
legs.
A modification of the above mentioned bridge circuit is to divide
the voltage to the leg containing the unheated resistance element
and then amplify or multiply the voltage output to cancel the division.
This generally requires the use of capacitor compensation to assure
stability of the gain stage of the amplifier which in turn slows
down the circuit response.
Still other solutions have placed the unheated resistance element
in the feedback circuit of an amplifier in the bridge circuit. This
requires a number of interactive function adjustments and a regulated
voltage reference.
In my invention illustrated in U.S. Pat. No. 4637251 I have
proposed a current bridge circuit which operates by balancing the
current flow between a heated and cold leg of a resistance bridge
which operates over a wide range of air flow to maintain a heated
resistor at a given increased temperature differential above a colder
resistor. In this case both the heated and cold resistor are temperature
sensitive resistors. The operative circuit as illustrated in FIG.
3 of my patent requires for its operation the precise balancing
of a large number of resistors. As such, in a production environment
this circuit may prove to be relatively difficult to adjust the
various resistors to compensate for differences in the temperature
coefficient of resistance of the hot and cold resistors and also
to modify these resistors to maintain a precise and desired temperature
differential of the heated resistor.
In contrast, the present invention provides for a relatively simple
and straight forward current bridge arrangement requiring very few,
if any, adjustments to a select few resistors to obtain the required
temperature differential and compensation for variations in the
temperature coefficient of resistance of the heated and cold resistors.
It is an object of the present invention to sense airflow by balancing
the current flow in a current bridge arrangement.
Accordingly the invention comprises a circuit comprising:
power driver means, adapted to be connected to a voltage source
for supplying current in response to a control signal;
means for maintaining a desired temperature offset between a first
temperature variable resistor and a second temperature variable
resistor comprising:
a current bridge (14) adapted to be positioned in an air-flow comprising
a sensing leg (16) including the first temperature variable resistor
(R.sub.h) and a first offset resistor (R.sub.20) and a compensation
leg (18) including the second temperature variable resistor (R.sub.c)
and a second offset resistor (R.sub.22),
current stabilizing means connected to the sensing leg (16) and
compensation leg (18) for causing the current flow in such legs
to be at a specified ratio, including a first amplifier (A.sub.1)
having its positive input connected to a first resistor (R.sub.19)
which is connected in series with the sensing leg and a second resistor
(R.sub.21), the output of the first amplifier is connected to a
first transistor (T.sub.s) which completes a series connection between
the compensation leg and the second resistor, these first and second
resistors are chosen to generate such specified ratio;
a first voltage divider is connected across the sensing leg, comprising
third and fourth resistors having approximately the same ratio as
the first and second resistors;
a second amplifier having a positive input connected to a junction
between the third and fourth resistors and its negative input connected
to sense the voltage across the compensation leg, and an output
of the second amplifier communicated to and used to regulate the
power driver means.
Many other objects and purposes of the invention will be clear
from the following detailed description of the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the Drawings:
FIG. 1 illustrates a circuit diagram incorporating the present
invention.
DETAILED DESCRIPTION OF THE DRAWINGS
As mentioned above the present invention relates to a current bridge
which includes means for balancing the current flow in various legs
of the bridge by selectively changing the resistance of a sensing
element. With reference to FIG. 1 there is shown a schematic diagram
of the present invention. A reference voltage source, V, is input
to the circuit 10 at node 12 and communicated to the collector
terminal of a Power transistor T.sub.p. The emitter of transistor
T.sub.p is communicated to a sensing or hot resistor R.sub.h, a
compensation or cold resistor R.sub.c, a voltage divider circuit
comprising resistors R.sub.7 and R.sub.8 and to another voltage
divider circuit comprising resistors R.sub.3 and R.sub.4. The circuit
10 comprises a resistive bridge generally illustrated as 14 comprising
a sensing leg which includes the sense resistor R.sub.h and an offset
resistor R.sub.20 and a compensation leg comprising resistor R.sub.c
and another offset resistor R.sub.22. In the preferred embodiment
of the invention, the resistors R.sub.h and R.sub.c are temperature
variable resistors in which the magnitude of the respective resistances
vary with temperature. Such resistors (R.sub.h and R.sub.c) are
preferably of a platinum or nickel variety having a temperature
coefficient of resistance (TCR) of approximately 3000-3300 parts
per million per degree centigrade (TCR of nickel 5000-5500 ppm/.degree.
C.). Ideally, the resistors R.sub.h and R.sub.c are chosen to have
identical temperature coefficients of resistance (TCR) and the identical
resistance at ambient or room temperature. In the sensing or heated
leg 16 of the bridge circuit 14 is the series combination of resistors
R.sub.h and R.sub.20. Resistor R.sub.19 is in series with R.sub.h
and R.sub.20. One terminal of R.sub.19 is connected to ground. The
common terminal between resistors R.sub.20 and R.sub.19 is connected
to the positive input of an amplifier designated as A.sub.1. The
first voltage divider comprising the series resistance combination
of R.sub.7 and R.sub.8 is connected across the emitter terminal
of the power transistor T.sub.p to the positive input of amplifier
A.sub.1. The output of amplifier A.sub.1 is connected to the base
of a transistor T.sub.s which is located in series with the compensation
or cold leg 18 of the bridge circuit 14. The collector terminal
of the transistor T.sub.s, and a terminal of R.sub.22 are communicated
to the negative input of an amplifier A.sub.2. The positive input
of amplifier A.sub.2 is connected to the common junction between
resistors R.sub.7 and R.sub.8. The emitter terminal of transistor
T.sub.s is communicated to the negative input terminal of amplifier
A.sub.1 and to ground potential through resistor R.sub.21. The voltage
appearing at the positive input terminal of amplifier A.sub.1 comprises
the controlled output voltage of the present invention which may
be communicated to a voltage following amplifier A.sub.4 as shown.
The output of amplifier A.sub.2 is connected to the positive terminal
of an optional amplifier A.sub.3 the negative input of which is
communicated to the positive terminal between the second voltage
divider 30 comprising resistors R.sub.3 and R.sub.4. The output
of amplifier A.sub.3 may be communicated via resistor R.sub.5 to
the base of the power transistor T.sub.p. The circuit illustrated
in FIG. 1 will operate adequately without amplifier A.sub.3 by driving
the power transistor T.sub.p directly or through a bias resistor
R.sub.5 from the output of amplifier A.sub.2.
In operation the circuit 10 maintains the temperature of the sensing
or hot resistor R.sub.h a predetermined level above the temperature
of the compensation or cold resistor R.sub.c. In the preferred embodiment
this temperature differential is approximately 80.degree. C. for
all air flow conditions.
During ambient conditions, the circuit, if powered, will operate
to generate the above desired temperature differential between R.sub.h
and R.sub.c. While it is desirable to have the sensing resistor
R.sub.h and compensation resistor R.sub.c identical (in magnitude
and having the same thermal temperature coefficient of resistance,
TCR) in practice this is most often not practical.
Resistors R.sub.20 and R.sub.22 provide an off-set control function
to move the initial temperature differential up or down, as the
case maY be, to generate the desired temperature differential. The
trimming of the resistors is performed during initial manufacture.
Typically this adjustment is done by trimming resistors R.sub.20
and R.sub.22. If the initial temperature differential is too small,
R.sub.20 is lowered (R.sub.22 increased). If the temperature differential
is too great R.sub.22 is lowered (R.sub.20 increased).
The circuit includes a second feature which is useful in compensating
for differences in the respective TCR's of R.sub.h and R.sub.c.
The ratio of R.sub.7 /R.sub.8 controls the temperature shift of
R.sub.h for a given temperature shift in R.sub.c such that for all
ambient temperature changes R.sub.h will track the changes in R.sub.c.
This compensation is called a gain G compensation or adjustment.
The following equations illustrate the relationship between the
sensing resistor R.sub.h, the compensation resistor R.sub.c, the
gain adjustment G and the temperature offset OS: ##EQU1##
As can be seen in the circuit of FIG. 1 the gain G adjustment is
a multiplier to R.sub.c compensating for a non-equal TCR relationship.
The off-set OS is shown as an effective resistance bias value which
yields an equivalent operating temperature differential (i.e. 80.degree.
C.).
In operation, the bridge circuit 14 will be balanced when R.sub.h
+R.sub.20 =R.sub.c +R.sub.22. Assuming that the TCR of resistor
R.sub.c is approximately 3000 ppm/.degree. C. to achieve the desired
80.degree. C. temperature differential R.sub.h + R.sub.20 should
be 2.4 ohms less than R.sub.c +R.sub.20 at room temperature. When
the circuit is initially activated R.sub.h + R.sub.20 is less than
R.sub.c + R.sub.22. In this case the bridge 14 is unbalanced. By
the connection of R.sub.19 and R.sub.21 to the unity gain amplifier
A.sub.1 the transistor T.sub.s will not turn on until amplifier
A.sub.1 generates a positive output.
In the circuit of FIG. 1 the ratios of R.sub.19 to R.sub.21 are
approximately 1/7 (approximately 14 ohms and 100 ohms). As such,
in an equilibrium condition the current (I.sub.21) through resistor
R.sub.21 is about one-seventh the current (I.sub.19) through resistor
R.sub.19 (i.e. I.sub.21 =1/7.times.I.sub.19). In addition the current
flow in the compensation or cold leg 18 and the sensing or hot leg
16 of bridge 14 are in the same ratio (i.e. 1:7). In air unbalanced
condition the power transistor will heat resistor R.sub.h until
R.sub.h +R.sub.20 =R.sub.c +R.sub.22. If R.sub.h and R.sub.c are
initially equal at room temperature (i.e. about 10 ohms) the bridge
14 will become balanced when R.sub.h is heated to approximately
12.4 ohms. At this point the voltage across the voltage divider
R.sub.7 + R.sub.8 is equal to the voltage across R.sub.h + R.sub.20.
The voltage across R.sub.7 is equal to the voltage across R.sub.c
+ R.sub. 22 and the output of amplifier A.sub.2 will decrease and
reduce the current delivered by the power transistor T.sub.p. R.sub.7
and R.sub.8 may be chosen to be approximately equal to R.sub.19
and R.sub.21 respectively.
As air flows across the sense resistor R.sub.h it will be cooled,
lowering its resistance. In this situation the output of amplifier
A.sub.2 goes high driving amplifier A.sub.3 causing more current
to flow through T.sub.p into the sense resistor R.sub.h heating
it to bring it back to its nominal value of approximately 12.4 ohms
and so maintains the desired temperature differential.
Similarly, if the air flow is such that R.sub.h is running too
hot the positive voltage to amplifier A.sub.2 is less than the voltage
supplied to its negative terminal. As such the output of amplifier
A.sub.2 will go low causing less current to be supplied from the
power transistor T.sub.p. As R.sub.h cools its resistance decreases
and brings the voltage across R.sub.7 back to a value to balance
the voltage across R.sub.c + R.sub.22.
Many changes and modifications in the above described embodiment
of the invention can, of course, be carried out without departing
from the scope thereof. Accordingly, that scope is intended to be
limited only by the scope of the appended claims. |