Understanding High-Pass Filters
Portable balancer & Vibration analyzer Balanset-1A
Balanset-1A is a portable, USB-powered vibration analyzer and balancer for one- and two-plane balancing of rotors in their own bearings. The Full Kit includes the interface unit, two vibration sensors, optical laser tachometer, reflective tape, Windows balancing software on a USB drive, magnetic stand, digital scale and transport case. Requires a compatible Windows computer with a free USB port; computer …
Vibration sensor
Vibration sensor for Balanset balancing devices, based on the ADXL335 accelerometer. Supplied with a 5 m cable as standard for measuring vibration during rotor balancing and analysis. A 10 m cable option is available.
Optical Sensor (Laser Tachometer)
Optical laser sensor for Balanset balancing devices, based on the modified HS2234 tachometer. Measures rotational speed without contact using a reflective mark on the rotor. Supplied with a 5 m cable as standard; a 10 m cable option is available.
Balanset-4
Balanset-4 is a four-channel vibration analyzer and balancing system for correction in one to four planes. Designed for cardan shafts and rotors supported on four bearings, it can also serve as the measuring system for a balancing machine. The kit includes four vibration sensors, an optical laser tachometer, USB interface, software, magnetic stand, scale and transport case.
Magnetic Stand Insize-60-kgf
Adjustable magnetic stand for positioning the laser RPM sensor in Balanset balancing kits. The switchable magnetic base provides up to 60 kgf of holding force on suitable ferromagnetic surfaces. Adjustable arms and joints help align the sensor with the reflective mark on the rotor.
Reflective tape
Silver self-adhesive reflective tape for the optical laser tachometer in Balanset balancing kits. Apply a small piece to the rotor to provide a reflective reference mark for rotational speed measurement. Supplied as a 1 m length for multiple setups.
Dynamic balancer “Balanset-1A” OEM
Balanset-1A OEM is the core measuring kit for one- and two-plane rotor balancing and vibration analysis. It includes the USB interface unit, two vibration sensors, optical laser tachometer, reflective tape and Windows balancing software on a USB drive. Compared with the Full Kit, the magnetic stand, digital scale and transport case are not included. Requires a compatible Windows computer with …
A high-pass filter (HPF) is a frequency-selective signal-processing element that allows vibration components above a specified cutoff frequency to pass through while attenuating components below the cutoff. In vibration analysis, high-pass filters remove low-frequency vibration (from unbalance and misalignment) so the analyst can focus on high-frequency content (from bearing defects, gear mesh, and electrical sources), and they eliminate DC offsets and low-frequency drift. It is the mirror image of a low-pass filter.
High-pass filters are fundamental to envelope analysis and general signal filtering (anti-aliasing, by contrast, is a low-pass function applied before digitisation), enabling extraction of diagnostic information from a chosen frequency range while rejecting unwanted low-frequency components that would otherwise mask or overwhelm the signals of interest.
1. Filter Characteristics
Three parameters define how any high-pass filter behaves: its cutoff frequency, its slope, and its underlying design type.
- Cutoff frequency (fc): the frequency at which the filter response drops to −3 dB (70.7% of the passband amplitude). Below fc frequencies are progressively attenuated; above fc they pass with minimal loss. The cutoff is chosen according to the application and the frequency content of interest.
- Filter slope (roll-off rate): the rate of attenuation below the cutoff, expressed in dB per octave or dB per decade. A 1st-order filter rolls off at 6 dB/octave (20 dB/decade) — a gentle slope; a 2nd-order at 12 dB/octave (40 dB/decade) — moderate; a 4th-order at 24 dB/octave (80 dB/decade) — steep. Higher orders give a sharper transition and better rejection but are more complex to implement.
The filter type determines the trade-off between sharpness and fidelity:
- Butterworth: maximally flat passband response.
- Chebyshev: sharper cutoff, but with ripple in the passband.
- Bessel: the best time-domain behaviour, with minimal phase distortion.
- Elliptic: the sharpest transition of all, but with ripple in both passband and stopband.
2. Applications in Vibration Analysis
Bearing defect detection
This is the most common application. A cutoff of typically 500–2000 Hz removes low-frequency unbalance and misalignment vibration, leaving the high-frequency impact signals generated by bearing damage. It is the first stage in envelope-analysis processing, which then demodulates those impacts to reveal the bearing fault frequencies.
Integration to velocity or displacement
When integrating acceleration to velocity or displacement, an HPF set at 2–10 Hz removes the DC offset and very low frequencies that would otherwise integrate into large drift errors. This step is essential for accurate low-frequency integration.
Sensor mounting resonance — a band-limiting task
An accelerometer mounting resonance — typically 3–10 kHz for a magnetic mount — can distort readings. Because this artefact sits at high frequency, a high-pass filter does not remove it: it is suppressed by limiting the upper frequency range with a low-pass (band-limiting) filter, or avoided by keeping the analysis band below the resonance. In envelope analysis, a resonance region may even be used deliberately as the demodulation carrier band. Sound sensor mounting practice complements the filtering.
DC-offset removal
A high-pass filter with a very low cutoff (0.5–2 Hz) strips out the DC component of a signal. This is necessary for proper signal processing, preventing FFT errors and integration drift.
3. Practical Implementation
Analog versus digital filters
Analog high-pass filters are hardware circuits inside the signal-conditioning chain. They operate in real time as part of sensor signal conditioning (the anti-aliasing stage that accompanies them is a low-pass filter) and have fixed characteristics once designed. Digital high-pass filters are software-based and applied in post-processing; their cutoff and order are adjustable, and they can be applied or removed after data collection. Modern analysers offer multiple digital filter options so the same record can be examined several ways.
Selecting the cutoff frequency
For bearing analysis, set fc well above the rotor-related components — typically a 500–1000 Hz cutoff. This removes 1×, 2× and other low-frequency machine components and passes the high-frequency structural-resonance region excited by bearing impacts. Note that the bearing fault repetition frequencies themselves (typically 50–500 Hz) lie below such a cutoff: they are recovered afterwards in the envelope spectrum by demodulating the high-frequency signal, not passed directly through the filter. For integration, set fc at 2–5× the lowest frequency of interest: too low allows drift, too high attenuates valid low-frequency components, with 2–10 Hz being typical for general integration.
4. Effects on Measurements
A high-pass filter changes the signal in three ways the analyst must keep in mind:
- Amplitude effects: frequencies below the cutoff are reduced, very low frequencies essentially eliminated, and frequencies well above the cutoff left unaffected; the transition region shows a gradual reduction rather than a hard edge.
- Phase effects: all filters introduce a frequency-dependent phase shift, which can alter the shape of the time-domain waveform. Bessel filters minimise this phase distortion, which matters when waveform timing is being interpreted.
- Waveform effects: the filter removes low-frequency baseline variations and centres the time waveform around zero, which can change its apparent character. It is therefore important to know what filtering was applied when interpreting a waveform.
5. Combining High-Pass Filters with Other Filters
High-pass filters rarely act alone. Pairing a high-pass with a low-pass produces a band-pass filter: the HPF blocks low frequencies, the LPF blocks high frequencies, and the combination passes only a middle band — exactly the selectivity needed to isolate a specific frequency range. In a full multi-stage processing chain, anti-aliasing (low-pass) is applied before digitisation, a high-pass removes DC, and a band-pass conditions the signal for envelope analysis; this sequential filtering builds up complex signal conditioning from simple stages. Where a single narrow component must instead be rejected, a notch filter is the complementary tool.
6. High-Pass Filtering in Field Measurement
In day-to-day field work, the right high-pass setting is what makes a faint bearing defect visible beneath dominant rotor vibration. A portable two-channel analyser such as the Balanset-1A measures the broadband signal needed for both balancing and diagnostics, and applying a high-pass stage before envelope analysis lets the engineer separate early bearing defects from the large 1× unbalance response on the same machine. Understanding high-pass characteristics — cutoff frequency, filter order, and the effects on amplitude and phase — is therefore essential for sound bearing analysis, reliable signal integration, and any task that calls for frequency-selective measurement.
