Mass flow rate detector of electrostatic type

A flow rate detector of the type comprising a corona discharge circuit to cause partial ionization of a fluid at a section of a fluid passage and a probe positioned downstream for detecting the amount of ions carried by the fluid as an indication of the fluid flow velocity. To achieve direct detection of mass flow rate even when the fluid exhibits fluctuation in its density, the detector includes a control circuit which serves the function of maintaining the intensity of a discharge current flowing through the corona discharge circuit constant.

BACKGROUND OF THE INVENTION 
This invention relates to a device for measuring mass flow rate of a fluid 
through partial ionization of the fluid. 
Concerning internal combustion engines, particularly of automotive use, 
there is much need of detecting mass flow rate of a fluid, which may be 
air, fuel, combustible mixture or exhaust gas, in order to perform various 
sorts of control with the aims of, for example, lessening pollutants in 
the exhaust gas and attaining improved fuel economy. 
Various methods are known for measurement of mass flow rate of a fluid, but 
there are great restrictions on the application of these methods to 
automotive engine systems. As a consequence, only a few types of methods 
have been the object of practical consideration for use in automobiles. 
(1) Measurement of pressure difference produced by constriction of fluid 
flow 
This method, based on the Bernoulli's law, is principally the measurement 
of pressure difference between two points respectively upstream and 
downstream of a constricted section such as a nozzle, orifice or venturi 
provided in a fluid passage. Since the Bernoulli's law applies only to a 
steady flow of a nonviscous fluid, the employment of this method in an 
internal combustion engine system in which a fluid flow subject to 
measurement comprises inherently a pulsative flow component requires that, 
as an inconvenience, a substantially steady flow be established by certain 
means such as suitable dampers. Also as a matter of practical 
inconvenience, it is necessary because of the fact that the pressure 
difference to be measured in the engine system is of a very small 
magnitude to use a highly sensitive pressure gauge such as a 
mechanical-electrical transducer, which is costly and in its accuracy 
tends to be influenced by the fluid temperature, external vibrations, etc. 
As an additional inconvenience, it is a requisite to this method that the 
specific weight of the fluid passing through the constricted section is 
known since, in a formula as a basis of this method, the mass flow rate is 
given as proportional to the square root of the product of the specific 
weight and the measured pressure difference. If the specific weight 
exhibits fluctuation, there is the need of making certain correction of 
the result of the pressure measurement. 
(2) Measurement of velocity distribution 
In this method, mass flow rate is found by detecting velocity distribution 
of a fluid flow within a section thereof and integrating the distribution 
with respect to the area of the section. The velocity distribution is 
detected by means of, for example, Pitot tubes or hot-wire anemometers, 
but it is not easy to achieve the detection minutely and nevertheless 
simply. Hot-wire anemometers are convenient to practical use, but there is 
a problem that errors are involved unless the fluid temperature is 
constant. Besides, this method too is required that the specific weight of 
the fluid be found by a separate means. 
(3) Regulation of sectional area of fluid flow to maintain a constant 
pressure difference 
This method, practiced as an air flow rate detector in electronic fuel 
injection systems for automotive engines, employs a device such as an air 
valve which constitutes part of a fluid passage and is capable of 
maintaining a pressure difference produced between two points respectively 
upstream and downstream of the device by continuously varying the 
effective cross-sectional area thereof. Flow rate is detected from, for 
example, an angle of rotation of a control element of the device needed to 
cancel a change in the pressure difference attributable to a change in the 
flow rate. To achieve accurate measurement of mass flow rate by this 
method, there is the need of accurately grasping the density of the fluid 
with corrections for pressure and temperature. 
Recently it has been proposed as a new method of detecting flow rate to 
make corona discharge in a fluid flow to cause partial ionization of the 
fluid and detecting the movement of a portion of the formed ions as an 
accurate indication of the velocity of the fluid flow or volume rate of 
the flow. This method is advantageous in that a compact flow meter 
including principally no mechanically moving element can be produced and 
that the flow rate is indicated by an electrical signal. An instrument 
according to this method serves as a mass flow rate detector if a fluid 
subject to measurement has a constant density, but when the fluid exhibits 
fluctuation in its density this instrument can no longer accomplish a 
highly accurate detection of the mass flow rate since a change in the 
fluid density causes a change in the number of ions formed by corona 
discharge (under a fixed discharge condition). 
With regard to internal combustion engine systems, particularly on 
automobiles, neither air nor fuel flowing therein is strictly constant in 
density, but until now it has been a usual practice in detecting flow rate 
of either air or fuel to assume that the density of the fluid is constant 
because of an extreme complication which arises if a change in the density 
should be taken into account. However, there is the need of accomplishing 
a very precise control of air/fuel ratio in the engine systems to meet 
severe requirements for cleaner exhaust gas and better fuel economy, 
meaning the need of highly accurate measurement of the quantities of air 
and fuel being admitted into the engine. Particularly an accurate 
measurement of the mass flow rate of air in the engine induction passage 
is an important requisite for success in realizing a fully satisfactory 
air/fuel ratio control. Accordingly there is an earnest demand for a mass 
flow rate detector which is suitable in construction for use in automotive 
engine systems and fulfils its function even when applied to a fluid of a 
fluctuating density. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a flow rate detector, 
as an improvement on the above described instrument of the corona 
discharge type, which detector can indicate a true mass flow rate even 
though a fluid subject to measurement exhibits fluctuation in its density 
and has a simple construction suited for practical application to 
automotive engine system. 
A flow rate detector according to the invention comprises a set of two 
electrodes spaced and disposed in a section of a fluid passage, a power 
supply for applying a high voltage to the electrodes to make corona 
discharge across these electrodes thereby to partially ionize the fluid, 
an ion detection electrode disposed in the fluid passage at a distance 
downstream from the corona discharge electrodes, and means for producing 
an electrical output representing the amount of ions collected by the ion 
detection electrode. In these respects the flow rate detector is of a 
known construction. As the improvement according to the invention, the 
flow rate detector further comprises a control circuit which has the 
function of maintaining the intensity of a discharge current flowing 
between the corona discharge electrodes constant by regulating the output 
voltage of the power supply. 
Since corona discharge by this device is made with a constant discharge 
current, a change in the density of the fluid leads to no change in the 
total number of ions formed by the corona discharge. However, the amount 
of ions carried by the fluid flow to the ion detection electrode depends 
not only on the velocity of the fluid flow but also on the density of the 
fluid. As a consequence, the voltage produced by the ion detection 
electrode always represents a true mass flow rate. 
Preferably, the control circuit is constructed so as to detect an actual 
intensity of the discharge current and, if the detected current intensity 
exhibits any deviation from a preset value corresponding to a standard 
density of the fluid, cancel the deviation through regulation of the 
output voltage of the power supply.

DETAILED DESCRIPTION OF THE INVENTION 
For better understanding of the invention, an explanation of a known flow 
rate detector of an electrostatic type will be made with reference to FIG. 
1 in advance of the description of preferred embodiments of the invention. 
In FIG. 1, the interior of a pipe 12 gives a fluid passage 10 through which 
a fluid, either gas or liquid, flows always in a direction indicated by 
the arrow F. Two electrode members 14 and 16 are fixedly disposed in the 
fluid passage 10 so as to be opposite to and spaced from each other in a 
direction generally normal to the direction F of the fluid flow and are 
connected to a DC high voltage generator 18. Either one or both of the two 
electrode members 14 and 16 may be placed directly on the inside of the 
pipe 12. When the pipe 12 is of an electrically conducting material, the 
electrode members 14, 16 and/or leads for them are electrically isolated 
from the pipe 12 by means of suitable insulators. The arrangement of the 
electrodes 14, 16 and the ability of the high voltage generator 18 are 
such that corona discharge occurs across the electrodes 14 and 16 when a 
high voltage (usually of several kilovolts) is applied thereto from the 
generator 18. At a certain distance downstream from the electrodes 14, 16, 
an ion detection electrode 20 is disposed in the fluid passage 10 and 
grounded through a load resistor 22. The illustration of this electrode 20 
as to take the form of net is only by way of example. The fluid passage 10 
has a uniform cross-sectional area over the distance between the corona 
discharge electrodes 14, 16 and the ion detection electrode 20. The 
tubular shape of the positive electrode 14 and the needle-like shape of 
the negative electrode 16 (inserted radially into the tubular electrode 
14) in the drawings are preferable but should be taken as exemplary. 
The flow rate detector of FIG. 1, i.e. the combination of a corona 
discharge circuit and an ion detection electrode, operates on the 
following principle. 
Corona discharge is made across the electrodes 14 and 16 while a fluid 
flows through the passage 10 to cause partial ionization of the fluid 
through the following phenomena. Upon application of a high voltage to the 
positive and negative electrodes 14 and 16, an electric field is produced 
between the electrodes 14, 16 so that electrons emitted by the negative 
electrode 16 are biased and accelerated towards the positive electrode 14. 
Collision of an electron having a sufficiently large energy with a 
molecule of the fluid causes ionization of the molecule, producing a new 
electron and a positively charged ion. The thus produced electron too is 
accelerated in the electric field and makes a collision with another fluid 
molecule to cause ionization. As a consequence, electrons directed to the 
positive electrode 14 exhibit an increase in number by geometrical 
progression with a corresponding increase in the number of positively 
charged ions which migrate towards the negative electrode 16. 
Meanwhile a portion of the electrons lose energy through successive 
collision with a plurality of molecules, and the collision of an electron 
in such a state with a neutral fluid molecule results in the formation of 
a negatively charged ion. This ion is less influenced by the electric 
field because of its larger mass than either an electron or a positively 
charged ion and hence tends to be carried away downstream by the flowing 
fluid without arriving at the positive electrode 14. The arrival of the 
ions carried by the fluid at the ion detection electrode 20 is recognized 
as a voltage developed across the load resistor 22. Insofar as the fluid 
has a constant density and the voltage for the corona discharge is 
constant, a constant number of ions are formed by the corona discharge and 
accordingly the voltage across the resistor 22, or the quantity of ions 
carried to the ion detection electrode 20, is proportional to the velocity 
of the fluid flow. A change in the velocity of the fluid flow does not 
accompany a change in the quantity of negatively charged ions in the 
electric field between the electrodes 14 and 16 but causes, for example if 
the change in the flow velocity is an increase, an increased amount of the 
negatively charged ions to flow towards the ion detection electrode 20. 
Thus, the instrument of FIG. 1 can detect the velocity of the fluid flow 
and hence serves as either a volume flow rate detector or a mass flow rate 
detector 30 so long as the density of the fluid is constant. 
However, the situation is different when there occurs a change or 
fluctuation in the density of the fluid. Lowering in the density means a 
decrease in neutral fluid molecules in a volume of the fluid, so that the 
electrons liberated by the corona discharge exhibit a longer mean free 
path and are accelerated more greatly in the electric field. Accordingly 
each electron has an increased chance of colliding with fluid molecules, 
resulting in that an increased number of negatively charged ions are 
formed in the electric field. On the contrary, a rise in the density of 
the fluid causes a decrease in the number of the negatively charged ions. 
When the fluid exhibits a fluctuation in its density, therefore, the 
voltage attributable to the collection of ions by the ion detection 
electrode 20 exhibits fluctuation even though the velocity of the fluid 
flow remains constant: the voltage has no longer a linear relationship 
with the velocity of the fluid flow. Thus, the instrument of FIG. 1 does 
not serve as a mass flow rate detector unless the density of the fluid is 
constant. 
Now an embodiment of the invention will be described with reference to FIG. 
2, wherein the corona discharge electrodes 14 and 16, the ion detection 
electrode 20 and the load resistor 22 are arranged in the same manner as 
in the instrument of FIG. 1 in association with the fluid passage 10. 
However, one (14) of the corona discharge electrodes 14, 16 is grounded 
via a resistor 24 of a sufficiently high resistance and is connected also 
to the input gate of a control circuit 26. In this instrument, a DC high 
voltage generator 28 for applying a high voltage to the electrodes 14, 16 
is of a type capable of varying its output voltage in response to an 
electrical control signal. The output of the control circuit 26 serves as 
this control signal and is supplied to an input gate 28a of the generator 
28. 
When corona discharge is made across the electrodes 14 and 16, a current 
flows between these two electrodes 14, 16 and the intensity of this 
current is represented by a voltage produced across the resistor 24. The 
control circuit 26 has the function of modifying its output, i.e. the 
control signal to the high voltage generator 28, to control the output 
voltage of the generator 28 such that the current flowing between the 
corona discharge electrodes 14 and 16 has constantly a predetermined 
intensity. 
While the density of the fluid remains constant, there occurs no 
fluctuation in the intensity of the aforementioned current and accordingly 
the output voltage of the generator 28 is maintained constant. In this 
situation, the instrument of FIG. 2 operates in the same way as the known 
instrument of FIG. 1 does. A constant number of negatively charged ions 
are formed by corona discharge across the electrodes 14, 16, so that the 
number of ions collected by the ion detection electrode 20 has a linear 
dependence on the velocity of the fluid flow. In other words, a voltage v 
detected across the load resistor 22 is proportional to the volume flow 
rate (will be represented by M) of the fluid. The detection of M implies 
the detection of the mass flow rate of the fluid so long as the fluid has 
a constant density. 
When, for example, the fluid exhibits lowering in its density while the 
velocity in the fluid flow remains constant, there occurs increase in the 
discharge current flowing between the electrodes 14 and 16, i.e. the input 
voltage to the control circuit 26. Then the control circuit 26 makes 
regulation or modulation of its output (the control signal to the high 
voltage generator 28) so as to lower the voltage applied to the corona 
discharge electrodes 14, 16 until the intensity of the current flowing 
between the electrodes 14, 16 returns to a predetermined constant. A 
constant intensity of the current flowing between the corona discharge 
electrodes 14 and 16 regardless of the density of the fluid means that a 
constant number of electrons travel from the negative electrode 16 to the 
positive electrode 14 and accordingly that a constant number of negatively 
charged ions are formed in the electric field produced by the corona 
discharge. In the case of, for example, lowering of the fluid density 
under a constant velocity of the fluid flow and a constant intensity of 
the current, there occurs a decrease, which is proportional to the 
lowering of the fluid density, in the quantity of negatively charged ions 
carried towards the ion detection electrode 20 since an increased amount 
of the negatively charged ions are attracted by the positive electrode 14 
in dependence on the magnitude of the lowering of the fluid density. The 
voltage v across the load resistor 22 in the instrument of FIG. 2, 
therefore, is proportional to the density (will be represented by d) of 
the fluid. Since the voltage v is proportional also to the volume flow 
rate M, the voltage v can be given as a function of the product (d .times. 
M) which implies the mass flow rate of the fluid through passage 10. Thus, 
the instrument of FIG. 2 serves as a mass flow rate detector which is not 
influenced by changes in the density of the fluid flowing through the 
passage 10. 
It suffices that the control circuit 26 is essentially a function generator 
whose output (the control signal to the high voltage generator 28) varies 
such that the output voltage (V.sub.c) of the generator 28 varies as a 
function of the voltage (V.sub.r) across the resistor 24 in a manner as 
shown in FIG. 3. 
For example, the control circuit 26 is constructed as shown in FIG. 4. A 
coil 30, as a part of the control circuit 26, is associated with the 
circuit connecting the high voltage generator 28 to the negative electrode 
16 for the detection of a current flowing between the corona discharge 
electrodes 14, 16. A current signal provided by this coil 30 is put into a 
comparator 34 via an amplifier 32. The output of the comparator 34 
indicates the magnitude of a deviation, if any, of the aforementioned 
current signal from a reference signal and is supplied to a current 
regulation circuit 36 the output of which serves as the output of the 
control circuit 26. The high voltage generator 28 has a boosting 
transformer 38 and a pulse generator 40 connected to the primary coil of 
the transformer 38. The output of the current regulation circuit 36 is 
supplied as a control signal to the primary circuit of the transformer 38 
to achieve the hereinbefore described control of the output voltage of the 
generator 28 by regulating the intensity of the primary current in the 
transformer 38. 
It is possible to use a Hall effect element 42, as shown in FIG. 5, in 
place of the current detecting coil 30 in FIG. 4. Still alternatively, use 
may be made of a photo-coupler 44, shown in FIG. 6, made up of a light 
emitting diode 46 included in the corona discharge circuit and a 
photosensitive transistor 48. 
FIG. 7 shows another embodiment of a mass flow rate detector according to 
the invention. The corona discharge electrodes 14, 16, the resistor 24 for 
the discharge current detection, the ion detection electrode 20 and the 
load resistor 22 are arranged in the same manner as in the instrument of 
FIG. 2. In this case, however, the high voltage generator 18 of the 
constant voltage type is used in the corona discharge circuit, and the 
instrument includes a correction circuit 50 to which are supplied both the 
voltage v across the resistor 24 and the voltage across the load resistor 
22. While the fluid has a constant density, the sum of the current 
represented by the voltage v and the current flowing through the load 
resistor 22 is constant. The correction circuit 50 has the function of 
detecting any deviation of the sum of the two input voltages and 
accomplishing an amplitude modulation of the output of the load resistor 
22 so as to compensate for the detected deviation.