Page 203 - Understanding Automotive Electronics

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6 SENSORS AND ACTUATORS

for weather forecasting. In the MAF, the hot-wire, or sensing, element is
replaced by a hot-film structure mounted on a substrate. On the air inlet side is
mounted a honeycomb flow straightener that “smooths” the air flow (causing
nominally laminar air flow over the film element). At the lower portion of the
structure is the signal processing circuitry.
The film element is electrically heated to a constant temperature above
that of the inlet air. The latter air temperature is sensed using a solid-state
temperature sensor (explained later in this chapter). The hot-film element is
incorporated in a Wheatstone bridge circuit (Figure 6.2a). The power supply
for the bridge circuit comes from an amplifier.
The Wheatstone bridge consists of three fixed resistors R , R , and R
3
1
2
and a hot-film element having resistance R HW . With no air flow the resistors
R , R , and R are chosen such that voltage v and v are equal (i.e., the
b
a
3
1
2
bridge is said to be balanced). As air flows across the hot film, heat is carried
away from the film by the moving air. The amount of heat carried away
varies in proportion to the mass flow rate of the air. The heat lost by the film
to the air tends to cause the resistance of the film to vary, which unbalances
the bridge circuit, thereby producing an input voltage to the amplifier. The
output of the amplifier is connected to the bridge circuit and provides the
power for this circuit. The amplified voltage changes the resistance in such a
way as to maintain a fixed hot-film temperature relative to the inlet
temperature.
The amplifier output voltage v varies with MAF and serves as a measure
c
of R . Typically the conversion of MAF to voltage is slightly nonlinear, as
m
indicated by the calibration curve depicted in Figure 6.2b. Fortunately, a
modern digital engine controller can convert the analog bridge output voltage
directly to mass air flow by simple computation. As will be shown in Chapter 7,
in which digital engine control is discussed, it is advantageous to convert analog
sensor voltages to a digital format within the solid-state electronics associated
with the sensor. This conversion is convenient since it eliminates the need for
an analog-to-digital converter, which can be relatively expensive (see Chapter
4).
One scheme for converting the analog output voltage to a digital signal
uses a device that is known as a voltage-to-frequency (v/f) converter. This
circuit is a variable-frequency oscillator whose frequency f is proportional to
the input voltage (in this case, the amplifier output voltage).
The variable-frequency output voltage (v ) is applied through an
f
electronic gate, which is essentially an electrically operated switch. Control
circuitry (also part of the sensor solid-state electronics) repeatedly closes the
switch for a fixed interval t. Then it opens it for another fixed interval. During
the first interval the variable-frequency signal from the v/f circuit is connected
to the binary counter (BC) (see Chapter 3). The BC counts (in binary) at the
instantaneous frequency of the v/f, which is proportional to the amplifier
output voltage v , which in turn varies with mass air flow rate.
f

190 UNDERSTANDING AUTOMOTIVE ELECTRONICS