Data processing: measuring – calibrating – or testing – Calibration or correction system – Fluid or fluid flow measurement
Reexamination Certificate
2001-08-30
2004-10-26
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Calibration or correction system
Fluid or fluid flow measurement
C702S024000, C073S204180, C073S204140
Reexamination Certificate
active
06810345
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a gas flow meter for automotive control and more particularly to a noise reduction circuit, to an adjustment circuit, to a reduction in the number of adjustment terminals and output terminals, and to an output circuit.
A gas flow meter for detecting an air flow in internal combustion engines has been in use. An example of the gas flow meter is a constant temperature control hot wire type gas flow meter described in the Journal of Fluid Mechanics, vol. 47 (1971), pp577-599.
FIG. 25
shows an outline configuration of a gas flow detection circuit DECT
1
applying the constant temperature control heat wire type gas flow meter. This gas flow detection circuit mainly comprises an operational amplifier OP
1
, a power transistor Tr
1
, a heating resistor (also called a hot wire) Rh, a gas temperature measuring resistor (also called a cold wire) Rc and resistors R
1
, R
2
and keeps the temperature of the heating resistor Rh constant at all times, i.e., keeps its resistance constant by maintaining a bridge balance using the operational amplifier OP
1
. As the gas flow increases, heat taken from the heating resistor Rh increases resulting in an increased heating current. Because this heating current is proportional to a voltage between terminals of the resistor R
1
, the measurement of this voltage can determine the gas flow. The voltage output produced by the current detection resistor R
1
is processed by an adjust circuit having a predetermined input/output characteristic so that the voltage output provides a predetermined signal characteristic required of the gas flow meter.
There is another gas flow detection circuit DECT
2
, as shown in
FIG. 26
, in which heat sensing resistors Ru, Rd for measuring gas flow temperatures are arranged upstream and downstream of the heating resistor Rh of the constant temperature control hot wire type gas flow meter so that they are influenced by heat from the heating resistor Rh. The resistor Ru on the upstream side is cooled by the gas flow to lower its resistance and the resistor Rd on the downstream side receives a gas flow heated by the heating resistor Rh to raise its temperature and therefore its resistance. This changes the potential at a connecting point between Ru and Rd and thus measuring this voltage can determine the gas flow.
Still another gas flow detection circuit DECT
3
as shown in
FIG. 27
is available, in which a total of four heat sensing resistors for measuring gas flow temperatures are arranged two upstream and two downstream of the heating resistor Rh of the constant temperature control hot wire type gas flow meter so that they are influenced by heat from the heating resistor Rh, and in which one pair of resistors Ru
1
, Rd
1
are serially connected in an upstream-downstream order and another pair of resistors Rd
2
, Ru
2
are serially connected in a downstream-upstream order to form a bridge and measure a potential difference between two connecting points. The resistors Ru
1
, Ru
2
on the upstream side are cooled by the gas flow to lower their resistances and the resistors Rd
1
, Rd
2
on the downstream side receive a gas flow heated by the heating resistor Rh, raising their temperatures and therefore their resistances. This changes the potential difference in the bridge and thus measuring this voltage difference can determine the gas flow.
The electronic circuits that adjust the output characteristic of a gas flow meter mounted on motor vehicles are subject to various surges and overvoltages, as specified in the International Standard Organization (ISO) 7637-1, 7637-3 standard and Japan automotive standard (JASO) D001-94. These standards are intended to prevent undesired operations or failures of electronic circuits due to surge voltages caused by ignition of engine, overvoltages caused by batteries stacked in two tiers at time of starting engine in cold environment, and high frequency noise caused by other electronic devices. On the other hand, the electronic circuits are constructed in the form of IC circuits for reducing the manufacturing cost and, in recent years, to meet the emission control requirements the gas flow meter is increasingly required to raise its precision in line with the sophistication of engine control functions. Further, because the service temperature range is as wide as −40° C. to 130° C., measures should be taken to prevent a possible change in output due to temperature variations.
For surges and overvoltages, a variety of overvoltage protection circuits have been in use. One such example is a protection circuit using a Zener diode ZD and a current limiting resistor R as shown in FIG.
28
.
The circuit of
FIG. 28
is one type of a commonly used constant voltage circuit in which a voltage applied to a connection terminal VBB for the battery causes a current to flow through the current limiting resistor R to the Zener diode ZD. When an overvoltage is applied, the voltage of the power supply terminal Vcc to various circuits is clamped by a Zener voltage of the Zener diode ZD to put an overvoltage protection into acton.
Further, JP-A-9-307361 proposes as a conventional technology an overvoltage protection circuit that uses an overvoltage detection circuit made up of a resistor and a Zener diode and a switching circuit made up of bipolar transistors.
The overvoltage protection circuit described in this official gazette is intended for protecting microwave FETs (field-effect transistors). When an overvoltage higher than a voltage sum of the Zener voltage of the Zener diode and the base-emitter voltage of the switching transistor is applied to the power supply terminal, the switching circuit is operated to cut off the load from the power supply line and thereby prevent the overvoltage from being impressed on the load.
The voltage outputs of the flow detection circuits DECT
1
-
3
in
FIG. 25
to
FIG. 27
need adjustments in zero point and span (output range) to produce the required sensor output characteristics. This adjust circuit is mainly an analog circuit at present but a higher precision adjustment is considered possible by using a digital circuit.
Table 1 shows comparison between an analog circuit and a digital circuit (“CMOS Analog Circuit Design Technique” published by Triceps (1998), compiled under the supervision of Iwata).
TABLE 1
Analog circuit
Digital circuit
The number
Few (about 20 pcs in
Many (2000 pcs in
transistors
multiplier)
8-bit multiplier)
Chip area
Small (few devices)
Large (many
devices)
Power
Low power
Large (many gates
consumption
consumption because
are switched)
of fewer devices
Clock
Low (determined by
Higher (½ of cut-
frequency
settling of
off frequency of
amplifier)
device)
Signal
High (about ½ of
Low ({fraction (1/10)} of clock
frequency
cut-off frequency of
frequency)
device)
Precision
Low (device
High (depending on
deviation, noise)
bit number)
Stability
Low (oscillation,
High
characteristic
variation)
Noise
Low (S/N)
Strong (large noise
resistance
margin)
Source: “CMOS Analog Circuit Design Technique” published by Triceps (1998), compiled under the supervision of Iwata
The analog circuit has a small size and a small power consumption compared with the digital circuit. But the use of such devices as resistors causes manufacturing variations and other variations due to aged deterioration, and thus the analog circuit has less precision and stability than the digital circuit. The digital circuit, while it is superior to the analog circuit in terms of precision and stability, has a larger circuit size and a larger power consumption. The rapid advance in the integrated circuit manufacturing technology in recent years, however, has enabled micro-fabrication and therefore reduced the circuit size and power consumption. The digital circuit is now finding many applications in various industrial fields. Example applications of a digital adjust circuit to the gas flow meter are found in Japanese Patent No. 3073089 and JP-A-8-62010 and JP-A-11-118552.
FIG. 29
shows comparison between an analog
Hanzawa Keiji
Katagishi Kenichi
Matsumoto Masahiro
Matsumura Takafumi
Sugiura Noboru
Barlow John
Crowell & Moring LLP
Dougherty Anthony T.
Hitachi , Ltd.
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