Vibration signal processing (Filters) :

Filters

Vibration signal from a transducer requires signal processing to produce the data that we need. The process usually involves filtering, setting sample rates and resolution, windowing, etc. It is important to understand what filters do and how they are used in the field of vibration. Filtering is a process that removes some frequencies from a signal in order to suppress interfering frequencies and reduce noise.

They are four types of commonly used filters:

1. Low pass filters
Low pass filters allow low frequencies to pass through. Low pass filters are the most common filter type because of the popularity in removing alias signals, and for other aspects of data acquisition and signal conversion.

2. High pass filters
High pass filters allow high frequencies to pass through. High pass filters are normally used in early bearing wear detection. A high pass filter is useful to block the high amplitude, lower frequencies to enable to “amplify” to the low amplitude levels of early bearing wear in the higher frequencies.

3. Band pass filters
Band pass filters allow frequencies within a band to pass through. Band pass filters transmit only those signal components within around a center frequency. Band pass filters are usually applied in situations that require extracting a specific tone, such as a test tone, from adjacent tones or broadband noise.

4. Band stop filters
Band stop filters block frequencies within a band from passing through. Band stop filters transmit all signals except those between specified ranges.

Ideally, filters would block unwanted frequencies and provide a clean cutoff and keep out of unwanted signals. However this is not the case. In reality, there is a transition region where some frequencies will be attenuated, but not blocked. The actual filter designs are shown in Figures. It can be concluded that it’s very important to understand filter types to enable us to look at the data that we are only interested in.