3. System and component architecture
In this section we will analyze the individual components of
the gas detection system in more details, focusing on the design
choices.
3.1. Sensor and relay nodes
The hazardous gas concentration can be measured using optical,
catalytic and semiconducting gas sensors [16]. The major problem
associated with the measurement of combustible gas concentration
in autonomous devices is the high power consumption associated
with the sensors. In this work, we have used a 2D semiconductor gas
sensor produced on an aluminum membrane [13]. This type of sensor
is characterized by high sensitivity, long lifetime and relatively
low energy consumption (see Table 1 ‘2D semiconductor sensor’).
Besides, the sensor has an ExsIU/ExdIIB+H2U degree of implosion
protection (see the protection requirements in [17]), which makes
it safe to use in a hazardous environment. The system works by
heating the sensitive layer of the sensor to a temperature of around
500 ◦C, and then measuring the conductivity of the semiconductor,
which is affected by the gas concentration. Typical applications [11]
use a continuous measurement mode in which the semiconductor
is kept at a constant temperature. In our system, instead, to further
decrease the energy consumption [18], the sensitive layer is only
heated when measurements are required, which occurs at regular
intervals, i.e., we replaced the continuous measurement mode with
the pulse of a PWM signal implementing a duty-cycle mode [19].
In addition to the gas sensor, the node shown in Fig. 2 includes
a microcontroller, a ZigBee module, and a battery power supply.
All components of the sensor node, as well as its operation modes,
were chosen with respect to minimal energy consumption. The full
block diagram of the sensor node is presented in Fig. 3. Note, that
the relay node is technologically identical to the sensor node except
that it does not include the on-board sensor for sensing capability.
The measurement and data processing is controlled by the 8-bit
ATmega168p RISC microcontroller [20]. The sensitive layer conductivity
and the heater temperature are measured with the AD
converter (ADC) built into the controller. The heater is driven by
a PWM signal with a switching frequency of 1 kHz. Using PWM it
is possible to change the heating profile depending on the environmental conditions and, at the same time, to avoid extra energy
consumption to supply an additional DAC chip, since the PWM signal
is software controlled. In order to let the heating current pass
through, the PWM signal is used to control a transistor switch. As
measurements are taken at regular intervals, functioning of the
measuring section of the circuit is not required in between the
measurements. In this case, the supply voltage of the measuring
section and the microcontroller ADC module are switched off to
reduce power consumption.