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Laser Doppler vibrometers, which are used to measure a multitude of the overall hard disk drive system’s and its sub-components’ dynamic properties, have proven themselves to be indispensable tools when it comes to developing computer hard disk drives. The vibrational properties of the delicate mechatronic or microelectromechanical systems of the read/write heads including the actuators and suspension can be determined in experiments only when using non-contact optical measurement methods such as laser vibrometry. Only methods such as this have the required resolution and dynamics to carry out modal and resonance tests for the purpose of determining and optimizing dynamic parameters and usage conditions. Laser vibrometers also help you to identify the causes of noise in the drive system during noise tests, so that you can then purposefully implement constructive measures to reduce the noise being emitted to below the perception threshold. Vibrometers have also proven to be highly efficient sensors for monitoring quality in production.

Identifying causes of noise in hard disk drives

In addition to ensuring capacity, access times, reliability and form factor, optimizing acoustic properties – or, in other words, eliminating perceptible interfering noises – is a primary objective when developing hard disk drives. Individual frequencies emanating from the hard disk drive’s electromagnetic circuit are a major source of noise.

Whistling noises in electric motors are caused by individual components being stimulated to produce vibrations during operation. Scanning vibrometry helps to systematically measure the drive’s and its components’ operational vibrations in terms of frequency, amplitude and deflection shape. The results allow the user to correlate the causes of the major interfering sounds very well with the vibration behaviour of a hard disk motor’s individual components. To reduce unwanted noises, you can then use the measurement results both to purposefully optimize the components’ proportions and their relative position to one another and to suppress the interfering sounds to below a specific perception threshold.

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Finite element analysis of hard disk drive vibration modes

(Laser Doppler vibrometry, being a non-contact and non-intrusive optical measurement method, has proven its worth as an ideal tool for conducting experimental modal analysis and FEM model verification operations on hard disk drive components. A high degree of importance has, in this regard, been attached to the rotating magnetic disks on the one hand and to the read/write unit with actuator, actuator arm and read/write head on the other. The undesirable vibrational properties of these components have a direct influence on a hard disk drive’s performance, reliability and service life.

The hard disk drive’s material and geometrical parameters are included in its FEM analysis. A scanning out-of-plane vibrometer measurement allows you to directly validate the model prediction for resonance frequencies and deflection shapes in a stationary condition and also shows you the deviations during operation that occur as a function of the rotational speed at the typical rotational speeds up to 15,000 rpm.

Documents

Noncontact modal testing of hard-drive suspensions using ultrasound radiation force

A Practical Example:

3D modal test on the RW head: Scanning vibrometers that perform three-dimensional measurements and simultaneously capture both the in-plane and the out-of-plane parts of the movement are used to conduct modal analyses on the read/write unit. You can export the data derived from this process directly using the universal file format for the purpose of both modal analysis and model updating.

HDD Schreibleseeinheit: 3D Schwinungsdaten

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Testing hard disk drives’ resonances

Hard disk drives – with their ever growing storage densities and shorter access times – require an extremely high level of stability as regards the read/write head’s location and positioning in relation to the disk drive interface. The flying height is a compromise between competing effects. A lower flying height enables better local resolution for read/write operations and thus a higher data density; meanwhile the risk of collisions with the medium grows at the same time. The flying height is just a few nanometers and very much depends on the ambient pressure due to the aerodynamic bearing. The aerodynamic bearing, however, has resonances that depend on the ambient pressure too and that may lead to instabilities.

Since the measurement process is both non-contact and non-intrusive, in this situation using laser vibrometers is the only way of measuring the response behaviour of the read/write head including its suspension following dynamic excitation. When performing resonance test measurements with single-point and scanning vibrometers, the frequency spectrum of the read/write head’s deflection is measured as a function of the ambient pressure. You can use this to identify critical conditions and then make constructive changes. The goal of the optimization process is to develop read/write units that respond robustly to resonances caused by aerodynamic excitation.

Documents

Article minimize disturbances by design

Resonanztest an Schreibleseeinheit einer Festplatte mittels differentieller Messung

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