Optical Karman Vortex Type - Toyota COMPUTER-CONTROLLED SYSTEM Training Manual

ELECTRONIC CONTROL SYSTEM - Air Flow Mete r
2 . OPTICAL KARMAN VORTEX TYP E
This type of air flow meter directly senses the
intake air volume optically . Compared to the
vane type air flow meter, it can be made smaller
and lighter in weight . The simplified construction
of the air passage also reduces inlet resistance .
This air flow meter is constructed as shown in
the following illustration :
Mirror
LED Leaf sprin g
From
W~'M
I To air
air
moo,
Karman
♦ intake
creaner ~9
v
n .toYO~ chambe r
Pressure-
directin
Mirro r
~ Pressur e
~ directing ape
rt
ure
OHP 1 3
Vo
rt
ex
/
___ . . 9 Phntntrancictn r
generator OHP 1 3
OPERATION AND FUNCTIO N
A pillar (called the "vo
rt
ex generator") placed in
the middle of a uniform flow of air generates a
vo
rt
ex called
a "Karman
vo
rt
ex" down-stream
of the pillar .
The frequency "f" of the Karman vortex thus
generated, the velocity of the air "V" and the
diameter of the pillar "d" have the following
relationship :
f - K x d
®
of a piece of thin metal foil (called a "mirror") to
the pressure of the vortexes and optically
detecting the vibrations of the mirror by means
of a photocoupler (an LED combined with a
phototransistor) .
LE D
r=^
Phototransisto r
Vo
rt
ex -
generato r
The intake air volume (KS) signal is a pulse
signal like that shown below . When the intake
air volume is low, this signal has a low
frequency . When the intake air volume is high,
this signal has a high frequency .
High
Voltage
signa l
Low
Low
Intake air volume
Hig h
OHP 1 3
ELECTRICAL CIRCUITR Y
Air flow mete r
Pillar (vo
rt
ex generator )
KARMAN VORTEX
OHP 1 3
Utilizing this principle, the frequency of the
vortexes generated by the vortex generator is
measured, making it possible to determine the
air flow volume .
Vo
rt
exes are detected by subjecting the su
rf
ace
Phototransistor
Engine EC U
OHP 1 3
21