Page 205 - Understanding Automotive Electronics

2735 | CH 6 Page 192 Tuesday, March 10, 1998 1:10 PM

6 SENSORS AND ACTUATORS

such that the v/f frequency were 1500 cycles/sec, then the BC count would be
the binary equivalent of 150. In mathematical terms, the BC count B is given
by the binary equivalent of
B = f t

where
B = BC count
f = frequency of v/f
t = duration of closure of electronic gate
After the engine controller reads the count, the BC is reset to zero to be ready
for the next sample. In actual operation, repeated measurements of frequency f
are made under control of the digital engine control module (see Chapter 7).
This conversion of voltage to frequency is advantageous in digital engine
control applications because the frequency is readily converted to digital format
without requiring an analog-to-digital converter.

Indirect Measurement of Mass Air Flow
Recall that Chapter 5 presented an alternative to direct mass air flow
measurement in the form of the so-called speed-density method. This method
computes an estimate of mass air flow from measurements of manifold absolute
pressure (MAP), RPM, and inlet air temperature. We consider first sensors for
measuring manifold absolute pressure.
MAP Sensor Concepts
Several MAP sensor configurations have been used in automotive
applications. The earliest sensors were derived from aerospace instrumentation
concepts, but these proved more expensive than desirable for automotive
applications and have been replaced with more cost-effective designs.
It is interesting to note that none of the MAP sensors in use measure
manifold pressure directly, but instead measure the displacement of a
diaphragm that is deflected by manifold pressure. The details of the diaphragm
displacement and the measurement of this displacement vary from one
configuration to another.

Strain Gauge MAP Sensor
One relatively inexpensive MAP sensor configuration is the silicon-
diaphragm diffused strain gauge sensor shown in Figure 6.3. This sensor uses a
silicon chip that is approximately 3 millimeters square. Along the outer edges,
the chip is approximately 250 micrometers (1 micrometer = 1 millionth of a
meter) thick, but the center area is only 25 micrometers thick and forms a
diaphragm. The edge of the chip is sealed to a pyrex plate under vacuum,
thereby forming a vacuum chamber between the plate and the center area of the
silicon chip.

192 UNDERSTANDING AUTOMOTIVE ELECTRONICS