Page 783 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 2 3. 1 Automotive instrumentation and telematics

23.1.7 Vehicle speed measurement

An example of a digital speed sensor has already been
described in Chapter 13.1 for a cruise control system. A
sensor of this type is assumed to be used for car speed
measurements. The output of this sensor is a binary
number, P, that is proportional to car speed S. This binary
number is contained in the output of a binary counter
(see Chapter 13.1). A block diagram of the in-
strumentation for vehicle speed measurement that uses
this digital speed sensor is shown in Fig. 23.1-15.
The computer reads the number P in the binary
counter, then resets the counter to zero to prepare it for
the next count. After performing computations and fil-
Fig. 23.1-14 Oil pressure sensor. tering, the computer generates a signal for the display to
indicate the vehicle speed. A digital display can be di-
rectly driven by the computer. Either mph or kph may be
selected. If an analog display is used, a D/A converter
the PDDL (see Fig. 23.1-1). These measurements serve
as the address for a ROM lookup table to find the oil must drive the display. Both mph and kph usually are
pressure limit. If the oil pressure is below the allowed calibrated on an analog scale.
lower limit or above the allowed upper limit, an output
signal is generated that activates the oil pressure warning 23.1.8 Display devices
light through the DEMUX.
It is also possible to use a proportional display of actual One of the most important components of any measuring
oil pressure if a cross-reference table, similar to the fuel instrument is the display device. In automotive in-
quantity table, is used. A digital display can be driven strumentation, the display device must present the re-
directly from the computer. An analog display, such as sults of the measurement to the driver in a form that is
the electric gauge, requires a D/A converter. easy to read and understand. The speedometer, ammeter,

Fig. 23.1-15 Vehicle speed measurement.

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