## 3 Condition Monitoring of the Impedance Bonds
### Condition Monitoring Device and Sensors Used for Monitoring the Impedance Bonds
#### 3.2.4 Digital Temperature Sensors  
The digital temperature sensor is used to acquire the PCB temperature. This sensor type is DS18B20, which is found inside of the device on the mainboard, and it offers temperature readings with a resolution from 9 bits to 12 bits (configurable). It has an alarm function with a non-volatile programmer at the upper and lower trigger points. The digital temperature sensor communicates with the microcontroller through a 1-Wire bus that, by definition, requires only a single data line (and ground) for communication with the microcontroller. The measuring range is in the interval (-55 \({}^{\circ}\)C... +125 \({}^{\circ}\)C), with an accuracy of \(\pm\) 0.5 \({}^{\circ}\)C. The power required for reading, writing, and performing temperature conversions can be obtained from the data line itself, without the need for an external power source. By using all of these sensors described above, the impedance bond parameters are acquired. These measurements provide valuable information about the impedance bond condition and allow, in most cases, accuracy in checking the presence or absence of anomalies. The software component is added to the hardware part of the device that involves the proper programming of the microcontroller, respectively reading, processing, and analyzing the acquired data by the hardware components of the device. For easier microcontroller programming, a connector has been introduced in the electric scheme, which allows In System Programming (ISP). The microcontroller software was designed to acquire the temperatures from the 4 NTC temperature sensors placed on each impedance bond terminal and one for its housing. The currents from each impedance bond semi-winding were acquired from two current transformers. The software was developed to transmit locally on the SD-card the monitored parameters or a local PC, and to display the measured temperatures on LCD. The microcontroller programming was done in the CodeVisionAVR environment, an ANSI C Compiler for Microchip AVR Microcontrollers. Upload the firmware in microcontroller was made using PonyProg, programming for serial devices. Acquisition of the impedance bond parameters can be performed in two ways: manually by selecting from the menu data acquisition the \"Data acquisition in file xxx1\" and automatically when exceeding the specified current threshold. The data acquisition period ranges from 1 to 10 s, and acquired data are stored on an internal SD card. They can further be downloaded to a computer or directly transmitted to PC by communication interfaces. The monitored parameters are recorded on SD card in CSV format, and can be downloaded to the computer for additional analysis. If the predetermined value was exceeded, the device launched an alarm signal indicating that one of the impedance bond parameters had passed into an unacceptable range. Finally, the device is capable of locally displaying all relevant data, and transmitting several types of alarms through GSM:  
* exceeding a minimum/maximum temperature limit for each monitored temperature point;
* exceeding the maximum current allowed by each half impedance bond winding;
* exceeding a maximum allowable imbalance between the currents on each rail;
* detecting the interruption of a connection cable. The monitoring device was tested and validated according to the internal instructions of the railway laboratory, using a test setup similar to that in Figure 6, at multiple current values. The results are based on repeated measurements, the number is sufficient to give uncertainty. For a test current of 150 A, the estimated expanded uncertainty (stated at the standard uncertainty of measurement multiplied by the coverage factor k = 2, which, for a normal distribution corresponds to a coverage probability of approximately 95%) is 4.94%. Table 2 gives technical specifications of the impedance bond monitoring and diagnosis device. It can be mounted on each impedance bond existing in the return circuit from electric traction.