Extreme measurements become routine
Tom Shelley reports on some of the engineering breakthroughs achieved using the latest laser interferometers
By exploiting the Doppler effect – by which car horns and train whistles appear to drop in pitch as they pass – it may be possible to make displacement measurements with claimed accuracies down to 2 picometres – at movement speeds up to 30m/s and over the near-DC to 30MHz frequency range.
Applied to laser interferometers, the method has been used to measure ripples in the sides of a stylus in an atomic force microscope and the operation of valves in Formula 1 engines running at 19,000 rpm.
Routinely deployed in the development of hard disk read-write heads, the technology is now being applied to applications as varied as studies of insect wing mechanisms – with a view to making tiny unmanned automated vehicles (UAVs) – and improving the performance of motorised toothbrushes.
Roger Traynor, who is the vibrometry and velocimetry specialist with Lambda Photometrics says: “In practice, this effect may be seen in waves in front of a vehicle or vessel moving through water. If the water is flowing against the moving vehicle, the waves are compressed and of shorter wavelength than those in static water. If the flow is in the direction of the vehicles, the wavelength is longer than in static water.”
The same type of effect occurs with light. On the cosmic scale of things, galaxies moving away from us produce light that is shifted towards the red end of the spectrum, and those moving towards us produce light shifted towards the blue end.
In the laboratory, the effect is used to make precise measurements using laser light in an interferometer. Eureka was sceptical about the 2 picometre accuracy claim, which seemed to fly in the face of Heisenberg’s Uncertainty Principle, but Traynor insisted: “I understand your concerns. Frankly, measuring down to these levels – effectively ripples in the molecular raft – certainly seems unbelievable, but it is real.”
He points to the example of “disk drive folk”, who need to locate a mechanical pickup – the read-write head of a disk drive – to 5nm in space using a servo mechanism that has a positioning capability an order of magnitude higher than that tracking requirement.
“You then need to measure that – which we do with our vibrometers – so you can see this is a real application giving real results,” he says.
Another example is monitoring surface waves – high frequency ripples that emanate from an impact, such as the striking of a hammer or shock wave from a laser pulse strike. Lambda has measured Lamb waves moving across the surface of bones, in response to rhythmic excitation from a piezoelectric probe. The first of five resolvable waves had a peak amplitude of around 1nm (well below the 632.8 nm wavelength of the laser in the vibrometer) and a wavelength appropriate to the excitation frequency.
“In a conventional laser interferometer, coherent light travels from a source and is separated into two paths – one for measurement and the other as a reference,” he says. “They are then recombined, and differences in the two paths show as interference fringing effects. By superimposing a known ‘carrier’ frequency modulation into the reference arm, relative plus/minus shifts can easily be measured to provide velocity information.”
The carrier frequency is selected to provide better velocity resolution than could be obtained by comparing the signal with the frequency of the laser light. By exploiting polarisation effects, it is possible to directly measure changes in displacement. This is the basis of Laser Doppler Vibrometers (LDVs). An associated technique, Laser Surface Velocimetry (LSV), uses converging beams split from a single laser source to measure in-plane or transverse motion. Again, Doppler frequency shifting is key, enabling speed and vibrational velocity to be measured down to zero or reverse speeds.
In studies of the human ear, the stapes bone (saddle and anvil) in the middle ear moves only about 2nm under normal sound level conditions. The hair cells in the cochlea in the inner ear move only around 100 picometers, which is about the size of an atom, at speeds of a few microns per second – yet they can easily be resolved using LDV.
At the other end of the scale, LDV is used to study valve movements in F1 engines. Effects that can be revealed include: not following cam profiles properly, leading to excess lift or bouncing off the top on the closing side of the cam; and seating bounce and/or overstressing of the valve stem at closure. Using a pair of differential LDVs is now the norm for developing such engines. One measures the valve motion while the second monitors and subtracts the background vibration of the cylinder block. Mushrooming distortion can be seen and measured if the two laser spots are targeted at the centre and edge of the valve head.
“Probably the most extreme example of an NVH study was the destructive vibration affecting the rear panels of the Thrust SSC car,” says Traynor. “The panels were subjected to high temperatures and about 170dB of noise, which was causing cracks in, and pulling rivets from, the titanium alloy panels. These were so severe that it was estimated that, when the record was captured, the vehicle only had a further three possible runs before it would be forced to retire.”
In a completely different sphere of activity, dentists commonly use descalers to remove plaque from teeth, which is more effective and gentler than traditional scrapers – particularly near and below the gum line. But early designs tended to fail prematurely due to a suspected mis-location of the critical vibration nodes and anti-nodes. Attempts were made to try and analyse the motion using high speed video microscopy, but the true behaviour was only revealed by the use of scanning LDV, first in one axis and then in 3D. The study, undertaken by the University of Birmingham School of Dentistry, showed that the descaler probes had a vibration anti-node – or hotspot – at the bend, rather than at the tip of the probe. This was the cause of the premature failures. Vibration data and modelling were subsequently used to adjust the descaler’s length, shape and natural vibrational frequency to improve efficiency, lifetime and effectiveness. The same methods have since been used on motorised toothbrushes to visualise and measure head movement, both in loaded and unloaded conditions.
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