Page 778 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Automotive instrumentation and telematics C HAPTER 23.1

Fig. 23.1-6 Data multiplexer.

The MUX and DEMUX selection is controlled by the method is satisfactory as long as the sample period is
computer. Note that in Figs. 23.1-6 and 23.1-7, each bit small compared with the time in which any quantity
of the digital code is multiplexed and demultiplexed. changes appreciably. Certain quantities, such as coolant
temperature and fuel quantity, change very slowly with
time. For such variables, a sample period of a few seconds
23.1.3 Sampling or longer is often adequate.
On the other hand, variables such as vehicle speed,
The measurement of any quantity takes place only when battery charge, and fuel consumption rate change rela-
the input and output switches (MUX and DEMUX) tively quickly and require a much shorter sample period,
functionally connect the corresponding sensor and dis- perhaps every second or every few tenths of a second. To
play to the computer, respectively. There are several accommodate the various rates of change of the auto-
variables to be measured and displayed, but only one motive variables being measured, the sample period
variable can be accommodated at any given instant. Once varies from one quantity to another. The most rapidly
a quantity has been measured, the system must wait until changing quantities are sampled with a very short sample
the other variables have been measured before that par- period, whereas those that change slowly are sampled
ticular variable is measured again. This process of mea- with a long sample period.
suring a quantity intermittently is called sampling, and In addition to sample period, the time slot allotted for
the time between successive samples of the same quan- each quantity must be long enough to complete the
tity is called the sample period. measurement and any A/D or D/A conversion required.
One possible scheme for measuring several variables The computer program is designed with all of these
by this process is to sample each quantity sequentially, factors in mind so that adequate time slots and sample
giving each measurement a fixed time slot, t, out of the periods are allowed for each variable. The computer then
total sample period, T, as illustrated in Fig. 23.1-8. This simply follows the program schedule.

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