3.3 A Single Sensor Node Hardware System
3.3.1 Sensor Node Energy Consumption
As described in Chapter 1, a wireless sensor node consists of a sensor module, a microcontroller, a transceiver and a power supply. In general, the energy consump- tion of a WSN as well as a sensor node is distributed into three main tasks: sensing, data processing and communication. The energy consumption of the sensing mod- ule is dependent on the nature of the applications which could be either sporadic sensing or constant monitoring. The complexity of the observation also has a great influence on the energy consumption of the sensor. The characteristics of some types of sensor popular used is shown in Table 3.1. It is observed that the current consumption of these sensors is in the range of 0.01mAto 1mAconsiderable small compared with that of other components such as microcontroller or transceiver which can be in the range of tenth mA.
Data processing is another activity that requires energy consumption of the sensor node. In microcontrollers using CMOS technology, the power consumption
Table 3.1: Example current consumption and related characteristics of sensors [102]
Sensor Accuracy Interchange
-ability
Sample rate [Hz]
Startup [ms]
Current [mA]
Photoresistor N/A 10% 2000 10 1.235
I2C temperature 1 K 0.20 K 2 500 0.15
Barometric Pressure 1.5 mbar 0.5% 10 500 0.01
Bar. press. temp. 0.8 K 0.25 K 10 500 0.01
Humidity 2 % 3 % 500 500-
3000
0.775
Thermopile 3 K 5 % 2000 200 0.17
Thermistor 5 K 10 % 2000 10 0.126
of data processing, (Pp), can be formulated as follows [103]
Pp =CtotalVdd2f+IleakVdd(N/f) (3.9) whereCtotalis the total switching capacitance;N is the number of clock cycles taken by the computation; Vdd is the supply voltage and f is the switching frequency.
The first term in Equation (3.9) is the main power consumption of the component and the second term is the power loss due to leakage current, Ileak.
Data communication consumes a remarkable amount of sensor node’s energy as this task involves both data transmission and reception. The average power consumption,Pc of the radio component is described as [104]
Pc=NT x[PT x(Ton−T x+Tst−T x) +PoutTon−T x] +NRx[PRx(Ton−Rx+Tst−Rx)](3.10) wherePout is the output transmit power;PT x,Rx is the power consumption of trans- mitter/receiver; NT x,Rx is the average number of times per second that transmit- ter/receiver is used; Ton−T x,on−Rx is the transmitter/receiver ON time interval and T is the transmitter/receiver START-UP time. The ON time interval
can be computed as
Ton = L
R (3.11)
where L is the data packet size and R is the data rate.
Thus, in order to minimize Pc, all the parameters listed in Equation (3.10) need to be handled efficiently during communication. For example, power saving can results from the reduction of the transceiver’s ON time interval achieved by transferring small size data packet at a high rate. In summary, the general strategies which can be used for sensor node power management are limiting energy wasted on any unnecessary tasks, turning off some parts or the whole circuit when they are not in use and minimize energy consumption to complete tasks. Table 3.2 shows the state of the sensor node using power mode scheduling algorithm [104] which saves energy by defining active and sleep mode of each component to use properly.
Table 3.2: Operation modes of the sensor node’s components States Processors Memory Sensor Radio
s0 Active Active On Tx,Rx
s1 Idle Sleep On Rx
s2 Sleep Sleep On Rx
s3 Sleep Sleep On Off
s4 Sleep Sleep Off Off
Depending on the requirement of the application, the decision of changing from a high power consumption state to a lower power consumption state is made to save more energy. Fig. 3.4 illustrates a scenario in which the sensor node tunes
hence it saves some energy.
Figure 3.4: Operation modes of a sensor node’s main components
At timet1, the sensor node is operating in states0 which has power consump- tion P0 and decides to change to state sk which has power consumption Pk < P0. It takes the sensor an interval τd,k to reach state sk so the energy consumed is τd,k(P0+Pk)/2, and then the sensor node spendsti−τd,k time units in this state. If remaining in the states0 , the total energy consumption isE0 =tiP0. Meanwhile, the energy consumed by the sensor node during the time ti −τd,k in state sk is (ti−τd,k−τu,k)Pk. Therefore, the total energy saving is
Esaved = tiP0−τd,k(P0+Pk)/2−(ti−τd,k)Pk
= ti(P0−Pk)−τd,k(P0−Pk)/2
= (ti−τd,k/2)(P0−Pk) (3.12)
And the overhead energy added due to the transition time of the sensor node from state sk to state s0 is
Eoverhead =τu,k(P0+Pk)/2 (3.13)
if the Eoverhead < Esaved or, equivalently if the time interval between two mode changing decisions is large enough
ti > 1 2
τd,k+P0+Pk P0 −Pkτu,k
(3.14) In conclusion, careful scheduling of such transitions has been considered from sev- eral perspectives which are regarded to application objectives, computation capa- bility of the microcontrollers as well as data communication protocols.