Military Communications and Information Technology: A Trusted ...

Chapter 1: Concepts and Solutions for Communications and Information Systems
95
Each processing chain or group of chains represents particular modules which
are designed like an intelligent sensors or on-board instruments connected to data
bus. Modules of intelligent sensors are based on 8-bit microcontrollers Atmel AVR.
More complex instruments, which carry out more complex signal processing like
radio communication, are based on FPGAs if any.
The signal processing block is essentially the only block that is different
from instrument to instrument. The result of the signal acquisition is transmitted
via CAN into the on-board system and since that moment can be used by other
instruments. Figure 2 shows some examples of signal processing blocks that we
recently created [16].
Third part of the instruments is the display block. Unlike conventional devices
that use stepper motors for displaying the flight parameter, our prototype is equipped
with a servo [2]. The main reason is with any doubt the price.
Figure 3. Servomotor as a gauge instrument
Servomechanism is driven by the microcontroller using a PWM (Pulse-width
modulation). The position of the servomechanism is related to the pulse length,
as shown the basic principle in Figure 3. The basic operational range of 180 degrees
can be enlarged by a simple gear mechanism. The precision is theoretically
limited only by a microcontroller timer/counter bit resolution. The precision for
16 bit counter is:
180 16 0.00275 (1)
2
This is sufficient precision to the show the flight parameter using a gauge.
Microcontroller reads the instrument corresponding value e.g. RPM, from CAN
and convert this information to the required length of the pulse [2]. What is more
important, the microcontroller can apply different modification to the displayed
signal e.g. linearization. The advantage is that the original signal still exists on
the CAN unmodified. The measured flight parameter is displayed also in the form
of number as a supplement, see Figure 1. The only thing that has to be modified