Accelerometers in Structural health monitoring
An accelerometer is a type of sensor that is commonly used in structural health monitoring to measure the acceleration and dynamic movement of a structure. It works by detecting movement and vibrations of a structure, which can be caused by factors such as wind, earthquakes, or other types of mechanical loads.
Accelerometers are often used in structural health monitoring because they can provide valuable information about the dynamic behavior of a structure. By measuring the acceleration of a structure, engineers can determine how it is responding to different types of loads and how it is likely to behave under different conditions. Knowing the dynamic behavior can help engineers detect changes in the global state of the structure.
There are many different types of accelerometers that can be used in structural health monitoring, each with its own unique characteristics and capabilities. Some accelerometers are very sensitive to measure induced vibrations from wind and loads, others have higher ranges able to detect movement of mechanical parts such as the wings on a windmill. Generally the sensors are made rugged and small such that they can be installed in outdoor conditions and tight spaces.
Overall, accelerometers are an important tool in structural health monitoring because they can provide valuable information about the performance of a structure and help engineers identify potential problems before they become serious.
Choosing the correct accelerometer
In order to choose the right accelerometer for the assignment it is important to know the required environmental protection, precision, maximum expected accelerations, noise to frequency levels and size of installation placements.
Environmental protection
Sensor in Structural health monitoring often need to be in operation for long periods (5+ years). If the proper environmental protection is not chosen it will lead to malfunctions, noise and errors at some point in the lifetime of the sensor. Generally a good rule of thumb for sensors installed outside for structural health monitoring is an IP rating of at least IP65 meaning a full protection against dust and a protection against water sufficient for rain and splashes.
In areas with high humidity and salt levels the material should also not be chosen lightly, typical sensor casings in aluminum will corrode fast on bridges around seawater. Here stainless steel can be a good choice for sensor casing.
Accelerometer precision
Accelerometer precision often ends up being a value dependent on the noise level on the sensor. If the noise at the frequency is larger than the signal which need to be measured the signal will be indistinguishable from the noise.
In Structural health monitoring it is often the precision at very low frequencies around 0 – 1 Hz which becomes the determining factor for the accelerometer. Many accelerometers will have a much higher noise rate around 0 Hz.
Maximum expected accelerations
When doing data analysis on accelerations the noise is the most important factors. Choosing the sensor range with the lowest levels still above the expected accelerations helps to minimize the noise and make the signal as distinguishable as possible. Signals from sensors are limited by there being a finite amount of points possible to measure between the minimum value and the maximum value. Choosing a smaller range means that the granularity of the signal will be finer and usually the noise level being smaller.
Noise to frequency levels
Noise is not just noise in accelerometers. Accelerometers in Structural health monitoring are used for monitoring vibrations, and depending on the stiffens of the structure these frequencies can be anywhere in the range from 0.1 Hz to 1000 Hz. When dealing with very low frequencies it is important to ensure that the noise level does not climb fast around 0 Hz. On large suspension bridges the hangers can be moving at a rate where it takes more than 5 seconds for them to complete a full cycle. Here a accelerometer will have to have low noise at the range of 0 Hz to 1 Hz. On the other hand a steel bridge with a train driving over it can have vibrations in the neighborhood of 200 Hz where the general precision becomes the important factor.