Evaluation of a Wireless Sensor Network with Low Cost and Low Energy Consumption for Fire Detection and Monitoring

Abstract

Wireless sensor networks (WSNs) may offer the opportunity to eliminate most of the extension cables and wires in digital systems, allowing operation far from any infrastructure. This opportunity coincides with a great increase in cost-effectiveness in an overall fire detection and monitoring system for forests, buildings or industrial sites. Our purpose is to evaluate this opportunity. After presenting the three main technologies for wireless communications to non experts, we retained the Zigbee protocol for this study. We then investigated whether the use of a WSN with this protocol is valuable for measuring heat quantities during a fire spreading over a vegetation fuel bed. Experiments are performed under both lab scale indoor and real outdoor conditions. The method consists of comparing temperatures and radiant heat fluxes gained with the wireless technology with those recorded at the same location through a wired data acquisition system. Delays due to the wireless radio communications are identified and explained. We also observe information loss for measurements performed in the fire front. Finally, we highlight that fires can be detected satisfactorily by WSN equipment in indoor and outdoor conditions. However, we also show that measurement accuracy obtained from wired systems cannot be obtained with the present wireless technology, and we do not recommend their use at the present time for fire monitoring and mitigation.

Appendix: The Timing of the Data Flow in Wired and Wireless Systems

The present appendix details for nonexperts the processing of information gained by a sensor through wired and wireless communication systems. In particular, it explains the differences in the time interval between wired and wireless solutions existing at each stage in the digital processing of analogue data.

Let us therefore consider the different stages in the sampling and recording of an analogue signal on a digital data logger for both DASs considered. In a wired system, the analogue signal coincides with a voltage of 0–12 mV inputted into the digital system by wires. It was converted into digital data by an analogue-to-digital converter (ADC) at a given sampling rate. In the case of a standard wired data logger (SDWL) such as the CR3000—the digital recorder of the WDAS solution—the time scale for converting the value of an electrical voltage is 96 ns when one value is read each second. This conversion rate is therefore extremely short compared to the sampling interval of the data (see figure A.1).

Let us now consider the process for the WSN system. The acquisition board MDA 300 reads a value in the continuous analogue signal each second. The sampling rate of each acquisition system in the ADC is therefore the same. However, in the WSN, after an ADC delay $\Delta t_{adc}^{WSN }$ (approximately 50 ms) [36], the data is then encoded for security and converted into a message involving packets. This occurs over a timescale of $\Delta t_{encoding}^{WSN}$ (approximately 20 ms). Next, the data is ready to be transmitted to the base station. At this instant, the data is timestamped by the MicaZ mote at the instant $t = t_{0} + \Delta t_{adc}^{WSN }\;+\;\Delta t_{encoding}^{WSN }\;+\;\varepsilon,$ where ε is the error due to the clock synchronization from a mote and the base station (ε cannot exceed 0.36 s in the Xmesh HP communication protocol because it is set to zero every 36 s). The process then involves a call from the mote to the base station each 125 ms (8 Hz). The base station listens to its neighbours at the same frequency. The data read by the MDA 300 remains the same as long as the transmission process fails, i.e. for eight attempts. Then, this data is erased and replaced by the new data flowing from the MDA 300 board. This process is summarised in figure A.1. The XMesh mapping is therefore set up to ensure stable communications between motes, ideally before the MDA reads the new following analogue value. The time scale during which the transmitter attempts to send its message is $\Delta t(j)_{transmission}^{WSN}$. This value depends on the number of attempts j for communicating with the base station $\Delta t(j)_{transmission}^{WSN} = j*125 ms$, where j = (1,2,…,8) is the number of attempts for radio transmission from a node to the base station. Finally, according to these processes, one can consider that the timestamp associated with an analogue record by the WSN MDA 300 acquisition board obeys the following model provided by the following equation:

$$ \Delta t_{communication}^{WSN} = \Delta t_{adc}^{WSN } + \Delta t_{encoding}^{WSN } + \Delta t(j)_{transmission}^{WSN} \pm \varepsilon $$

(A.1)

It is important to observe that the maximal transmission rate of a single node (time of 300 ms in Xmesh HP mode) is therefore estimated on the assumption that the radio transmission cannot fail more than two consecutive times (j = 2 leads to $t = t_{0} +$ 300 ms). However, this assertion is incorrect in real conditions, where interference may occur and force the number of attempts to be greater than two. As a theoretical distance for data transmission, one can observe that the radio transmission rate in the WSN cannot be set up with the present commercial system.

The model in Eq. (A.1) will be exact for a single node-to-base-station communication when it is evaluated on the record of each packet and when the process of decoding information and the flow to host are also described. In a multiple-node configuration, this model remains reliable, but another time interval $\Delta t_{multiplexing}^{WSN }$ appears. This is due to the multiplexing of incoming data at the base station. The network of an efficient WSN is able to transmit data ‘continuously’, with a base station which starts to read a new message, segmented in packets, before the end of the transmission of previous packets. This will delay the delivery of the acknowledgement, which completes the arrival of a full message at the base station.

This model explains why a delay of approximately 2 s to 23 s may exist between wired and wireless signals in the present experiments.

Incoming upgrades of the Zigbee protocol IEEE 802.15.4e should introduce a fast communications class, where $\Delta t_{communication}^{WSN}$ is expected to approach $\Delta t_{communication}^{SWDL}$, which will allow us to use a WSN as a data logging system.