# Star Type Wireless Sensor Network for Future Distributed Structural Health Monitoring Applications
## 1 Introduction
### Motivation for Experimental Work  
Attempts to understand the tower movement through finite element models were unsuccessful not due to the lack of computational power but due to the difficulty of properly modelling the problem as seen in Section 1.2. It is clear that experimental work is required to capture the real dynamics of the tower and the bells. An experimental investigation into the link between forces from bell ringing and tower movement had never been attempted. In previous experimental studies, for example, in a study of 19 bell towers in the North East of England, there was no correlation found between the maximum displacement of bell towers due to ringing and the tower height [7]. Instead, the maximum displacement of the tower depended upon its natural frequency [7]. Slender towers tend to bend to produce displacement at the top of the tower; however, shorter towers could rock in their foundations to produce equally large displacements [7]. This further indicates that experimental work is required to assess whether the movement of any given tower can be considered safe. The gained insight into the interaction of bell forces that cause the movement could be used to establish criteria for bell tower safety. Currently, the ratio of maximum sway to tower height is accepted as the most effective measure of the safety of tower motion [8]; however, this criterion is somewhat arbitrary. A value under 200 \(\times\) 10\({}^{-6}\) was deemed acceptable, and it was generally agreed that bell ringers would be reluctant to use a tower if the acceleration in the ringing chamber was greater than 50 mm/s\({}^{2}\)[8]. Nonetheless, the primary cause of movement was not addressed in [8] underlining the need for further experimental work.  
In addition to understanding the tower dynamics, measuring the natural frequencies of bell towers regularly could help identify structural damages. A definite sign of damage occurring in a bell tower was the decrease of the natural frequency of the tower vibration between two annual measurements [1].  
Total ringing time also has a large influence on the tower motion [9]; therefore, it would be advantageous to measure an entire session and review the data to see the oscillation build up over time. Large oscillations have the potential to develop during long sessions because in the English style, bells are rung in precise repeating patterns known as methods [2]. This effect is not observed in Continental Europe where the bells are rung in a random fashion [10]. It has been proven that the third harmonic is the most significant for the system used in Continental Europe [11]. In English full circle ringing, the contribution of the higher harmonics becomes much more significant, which means that simulations or experimental work are required to investigate the interaction of the bells [9]. Simulations of bell towers are problematic, because as previously mentioned, it is extremely difficult to develop an accurate model because of the simplifications and assumptions made about the building materials.  
Experimental measurements in the field, on the other hand, give valuable and inherently more valid insight into the causes of tower movements and lead to suggestions about how to reduce them, and the sensor network described in this work makes these measurements possible.