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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a fuel level measuring method for use in
an automobile and, more particularly, to a fuel level measuring method
which may be implemented with a microcomputer.
As disclosed in U.S. Pat. No. 3,842,673 to John W. Riddel, in a
conventional fuel level meter for an automobile, the fuel level in a fuel
tank is converted into an output signal by means of a sensor having
winding type resistor with a sliding contact. With a predetermined
resistance value of the sliding resistor, a current coming from a constant
voltage source is controlled to cause a pointer to point to the fuel level
measured. Practically, the shape of the fuel tank is not uniform and the
fuel level and the moving angle of the sensor are not always in a linear
proportional relation to each other. Therefore, some adjustment must be
made to the moving angle and the output resistance of the winding
resistor, so as to set up a linear relation between the fuel level and the
fuel level value indicated by the pointer.
The total of the errors accumulated through the signal process ranging from
the sensor to the pointer is great. For this reason, in actual practice,
the indication of the fuel level by the pointer is a mere approximation of
the actual fuel level. On the other hand, a driver requires information
concerning the fuel level in the fuel tank with the best possible
accuracy, since he uses such information more frequently, as compared with
other information for driving the automobile.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a fuel level
measuring method which provides an indication of fuel level with a high
accuracy, eliminating the above-mentioned disadvantages.
In brief, the fuel level measuring method of the present invention
previously stores data of the sensor outputs and the fuel level in a fuel
tank in a memory of the computer, and computes an accurate fuel level from
a mean value of the outputs from the sensor within a given period of time.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a system for a fuel level measuring method
according to an embodiment of the present invention.
FIG. 2 is a graphical representation of an output characteristic of a gauge
used in the system shown in FIG. 1.
FIG. 3 is a main routine of a microcomputer used in the system in FIG. 1.
FIG. 4 is a flow chart for processing an interrupt.
FIG. 5 is a flow chart for computing a mean value in the flow chart shown
in FIG. 4.
FIG. 6 is a graphical representation for illustrating a timing relation
between a fuel level and the interrupt.
FIG. 7 is a flow chart illustrating another computation of the mean value.
FIG. 8 is a graph for illustrating an input/output characteristic of a
converting ROM for converting digital data into another value.
FIG. 9 is a block diagram of another embodiment of the fuel level measuring
apparatus according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1, reference numeral 1 designates a fuel tank; 2 a float; 3 a fuel
level sensor. The fuel level sensor 3 has a sliding contact 31
interlocking with the float 2. An output signal from a middle point of the
resistor 312 is coupled by way of the contact 31 to one of the input
terminals of a voltage comparator 32. The fixed resistor member 312 is
constructed as a thick film resistance element. A common connection point
of a plurality of resistors 321 to 322 is connected to the other input of
the voltage comparator 32. The output terminal of the comparator 32 is
coupled with an input/output (I/O) circuit of a microcomputer 40. The
microcomputer 40 includes a microprocessor (MPU) for performing an
arithmetic operation and its related control, a read only memory (ROM) for
storing a program, and a random access memory (RAM) for setting up or
arranging data. Output registers R1 and R2 of the input/output port (I/O)
are coupled at respective bit positions thereof with resistors 321 of
which the values are weighted by a binary code. The other ends of the
resistors 321 are commonly grounded by way of resistor 322. The voltage
comparator 32, the microcomputer 40, and the resistors 321 and 322
cooperate to form an analog to digital converter. An oscillator 50 is
comprised of a piezoelectric resonator 51, capacitors 52 and 53, and a
circuit section 54. A speed sensor 60 is comprised of a magnet 61 rotating
in accordance with vehicle speed, and a lead switch 62. Reference numeral
63 designates a capacitor and 64 designates an inverter. An external
switch 70 is used for selecting a desired item or items to be displayed on
a display unit 80.
An output register R3 of the microcomputer 40 is used for driving the
display unit 80. The output thereof is applied to a BCD (binary coded
decimal) input of a decoder driver 81 in the form of a latch. The display
unit 80 is designed to display three digits, capable of displaying down to
the first decimal place. The output signal of the fuel level sensor 3
nonlinearly varies with the level of the residual fuel, as shown in FIG.
2. By means of the nonlinearity, a change in low fuel levels is preferably
plotted with a detailed scale in order to improve the resolution in
reading the fuel level.
The general flow charts of the microcomputer are as shown in FIGS. 3 and 4.
FIG. 3, illustrating a main routine, counts output pulses from a car speed
sensor 60. During the course of executing the main routine, a timer
interrupt is issued at fixed time intervals. Upon generation of the
interrupt, "0" is applied to the output register R1 of the analog to
digital converter, as shown in FIG. 4. Then, it is judged whether the
output signal from the voltage comparator 32 is "0" or "1". If it is "0",
"1" is added to the contents of the output registers R1+R2. Again, the
comparison is performed. Through repetition of the above-mentioned
operations, when the output signal of the voltage comparator becomes "1",
that is, when the output of the sensor 3 is equal in level to the terminal
output of the resistors 321, the contents of the output register are
stored in the memory. In response to the next interrupt, the data
resulting from the same operation as the above is compared with the data
stored previously. The larger of the two data items is stored in a memory
location provided exclusively for it. The smaller one is stored in another
memory location provided exclusively for it. In this way, the data
obtained through generation of the interrupts is stored and a mean value
of that data is calculated.
The position on a scale indicated by the sensor (fuel gauge) exactly
corresponds to the value displayed by the display, in the state where the
car is at a standstill. When the car is running, the liquid surface
constantly undulates due to the inequality of the load surface, or
deceleration or acceleration of the car. Therefore, unless some
countermeasure is taken for the undulation of the fuel level, the
displayed value always fluctuates, so that a driver finds it difficult to
read the scale of the fuel level. This is the reason why the mean value of
those varying indications is necessary. Because of the liquid nature of
the fuel, its level changes substantially sinusoidally with time, as shown
in FIG. 6. Accordingly, a flow chart as shown in FIG. 5 is preferably used
for the mean value calculation in the flow chart shown in FIG. 4. A
program flow for obtaining a position on the scale of the sensor, i.e. the
A/D conversion, is repetitively performed with relatively short time
intervals S, e.g. 5 msec. In this case, the present converted value is
compared with the preceeding one for each converting operation and the
larger value and the smaller value are stored separately. For a period
from a fixed time point to a time point at which the number of interrupts
reaches a given value, that is, for a fixed time period T (1 to 2 minutes,
for example), a mean value X.sub.mean is calculated from the stored
maximum values X.sub.max and minimum values X.sub.min, and this mean value
calculated is converted into a value which is used for display of the fuel
level as a corrected value.
The use of the mean value is still insufficient for obtaining the real
correct one. Generally, the liquid level undulates substantially
sinusoidally with time, but sometimes changes which are extremely high or
low occur, as indicated by a in FIG. 6. Those abnormally high and low
level changes of the fuel must be regarded as noise and therefore
neglected. In this respect, the mean of the values representing the second
or third magnitudes rather than the maximum and the minimum values,
provides a more correct fuel level. A major part of the program flow to
realize this approach is illustrated in FIG. 7.
Another approach to secure the correct fuel level is that a difference
between a value X0 currently being displayed and a value X1 to be
displayed is calculated and when the difference exceeds a given value, the
contents of the display is left unchanged. Assume that a normal running of
a car never experiences a situation that the X1 changes relative to the
X0, exceeding by 5% or more during the period T, for example. On this
assumption, the approach designs the apparatus so as not to display such
data of the relative change of 5% or more. The judgement of such, if there
is a relative change of 5% of more, is made, following the mean value
calculation in the flow chart shown in FIG. 5.
The data thus obtained is proportional to the output signal from the sensor
3. Therefore, it is nonlinear with respect to the level of the fuel.
Therefore, the data obtained must be converted into a value which is
linearly proportional to the actual amount of the residual fuel. To this
end, a converting table is prepared by using a read only memory (ROM)
within the computer. The data obtained is converted, in accordance with
the contents of the converting table, into an appropriate value which in
turn is applied to the display unit. The input/output characteristic of
the converting table is as illustrated in FIG. 8 and, as shown, the scales
in the low input region are divided into smaller sections.
As described above, a thick film resistor is used for the fixed resistor
312 in the sensor of the fuel level measuring apparatus. As a result, an
output signal accurately representing the fuel level measured may be
obtained, regardless of a shape of the tank. For example, a resolution in
reading values of the low fuel levels may be improved in a manner that in
a region where the fuel level is low, a rate of change of the resistance
with respect to a level change is set to be large. Additionally, since the
use of a function trimming method is allowed, the accuracy is further
improved.
Moreover, any suitable converting table may be prepared into the ROM of the
microcomputer. This indicates that a single system for measuring a fuel
level is applicable for a great variety of cars. Accordingly, this
apparatus is very economical.
Additionally, the apparatus has a good accuracy in low fuel levels about
which a driver is most worried. This ensures relaxed driving of the car.
The present invention has another useful effect. Since the automobile
necessarily sways when running, the liquid level of the fuel tank always
undulates. Because of the liquid nature of the fuel, however, fuel level
regularly undulates with substantially equal upper and lower magnitudes,
with respect to a liquid level in a standstill state. Therefore, the
correct data of the present residual fuel amount may be obtained by
calculating a mean of the maximum and minimum values of the liquid level
for a given period of time.
The data of the residual fuel thus obtained is displayed by the display
unit 80, and may also be used for calculating a specific fuel consumption
performed within the computer. In the later case, the result of the
calculation may be displayed to the driver.
In another embodiment of the present invention, the converting table may be
prepared without using the ROM in the computer. As illustrated in FIG. 9,
the control of the microcomputer is applied only to the analog-to-digital
converter. A ROM 410 may be used as means for converting the output of the
converter.
More specifically, in an automobile mounting a fuel level measuring
apparatus of the invention, an accurate amount of fuel is supplied to a
fuel tank, through a high precision fuel flow meter 100. The address of an
EPROM 410 is designated by the microcomputer 40. With respect to the input
data into the address designated, the flow of the fuel metered by the flow
meter 100 is written by a writing device 110. Once so done, a relation
between a fuel level and its display value is unconditionally determined,
regardless of the shape of the tank and the accuracy of the sensor. Of
course, this method is effective only when the car is at a standstill.
When the car is running, the above-mentioned method must be employed for
forming display data. If an electrically writable non volatile ROM, or
EPROM or EAROM, is used for the ROM 410, the write operation with a
relatively high accuracy for each car is ensured.
As described above, the present invention may provide a fuel level
measuring method and apparatus for this purpose. Thus, the present
invention has a great useful effect from the viewpoint of technology,
economy and reliability.
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