Page 324 - Understanding Automotive Electronics

2735 | CH 9 Page 311 Tuesday, March 10, 1998 1:24 PM

AUTOMOTIVE INSTRUMENTATION 9

Figure 9.14
Oil Pressure Sensor

FPO

During the measurement time slot, the oil pressure sensor voltage is
sampled through the MUX switch and converted to a binary number in the
A/D converter. The computer reads this binary number and compares it with
the binary number in memory for the allowed oil pressure limits. The oil
pressure limit is determined from load or crankshaft speed measurements that
are already available in the engine control system. These measurement data
can be sent to the instrument subsystem via the PDDL (see Figure 9.1). These
measurements serve as the address for a ROM lookup table to find the oil
pressure limit. If the oil pressure is below the allowed lower limit or above the
allowed upper limit, an output signal is generated that activates the oil
pressure warning light through the DEMUX.
It is also possible to use a proportional display of actual oil pressure if a
cross-reference table, similar to the fuel quantity table, is used. A digital display
can be driven directly from the computer. An analog display, such as the electric
gauge, requires a D/A converter.
VEHICLE SPEED MEASUREMENT

Digital measurement of An example of a digital speed sensor has already been described in
vehicle speed is possible Chapter 8 for a cruise control system. A sensor of this type is assumed to be
using a binary counter used for car speed measurements. The output of this sensor is a binary number,
and a sensor having an P , that is proportional to car speed S. This binary number is contained in the
output signal frequency output of a binary counter (see Chapter 8). A block diagram of the
that is proportional to instrumentation for vehicle speed measurement that uses this digital speed
speed. sensor is shown in Figure 9.15.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 311