## 2 Wireless Corrosion Monitoring Sensor and Network
### Corrosion-Detecting Sensor Mote Design  
In designing the corrosion sensing mote platform, we have to address the following three problems: (1) what sensing device will be used in corrosion signal detection, (2) which kind of wireless mote is appropriate for our objectives in terms of cost and extensibility, and (3) how to connect the corrosion-detecting device to the wireless mote. We will present the designed corrosion-detecting devices in the later sections. Next, we will introduce the MicaZ mote in use and the designed circuit connecting the corrosion-detecting device with the MicaZ mote.  
As a general wireless embedded platform, the MicaZ mote can not only measure 0-3 V with 0.01 mV resolution, but also be extended to monitor diverse other signals (such as temperature, humidity and vibration, _etc._), so we used the MicaZ mote in our test-bed to support a closer monitoring of corrosion of RC structures. In our design, we consider mainly two issues related to the WSN platform: one is how to connect the corrosion-detection device to the MicaZ mote, and the other one is how to implement a wireless communication algorithm to effectively collect the corrosion data. Figure 1 shows the architecture of our test-bed. The sensing layer consists of a set of sensor motes, which will self-organize into a wireless network. The laptop, data server, or other Internet-accessing devices belong to the gateway layer which is responsible for collecting all the data sent by the sensing layer. The back-end application layer provides a visual interface for users as well as domain experts. As long as users can access the Internet, they can browse the data and the corresponding analysis results, or even download them.  
Even though the MicaZ mote is fitted with a 51-pin connector for external sensing devices, it cannot directly read the signal output of our corrosion-detection devices. This is because MicaZ'sanalog-to-digital conversion (ADC) interface (channel) cannot directly measure the current. However, the corrosion current is what we are very concerned about in corrosion evaluation. We therefore designed a connecting circuit between the corrosion-detecting device and the MicaZ mote, and the circuit functions by transforming the current output of the corrosion events into a voltage that can be measured by the MicaZ. We have to map each voltage reading of ADC channel of MicaZ into a current reading. Figure 2 shows the kernel part of the circuit [28]. The accuracies of the potential and current measurements are \(10^{-5}\) V and \(10^{-7}\) A, respectively.  
The intermediate circuit uses an Analog Devices OP-amp of AD8603 AUJZ, which is a low-power, rail-to-rail amplifier with the working current less of than 50 \(\upmu\)A. Since the circuit needs a very low power level, the total power requirement of the circuit and the MicaZ mote can be kept within 600 \(\upmu\)A, effectively prolonging the system lifetime. Additionally, an array of resistors is used to identify the  
Figure 1: The architecture of the wireless corrosion monitoring system.  
Figure 2: Schematic of the connecting circuit for the wireless corrosion sensor.  
corrosion potentials, and this makes the circuit be able to measure at the 0.1 uA level, the precision of which is completely sufficient for our scenario.