I relies the displacement is very small and with up to a decent frequency.

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Here is details of the Magnet data Magnet poles North/South over the height(h) = 1.5 mm Coating: Nickel Length(a) = 8.5 mm Width(b) = 2 mm Height(h) = 1.5 mm

Material/grade: 48H max.operation temperature = 120 °C Flux density inside the magnet = 1.37 Tesla = 13700 Gauss Flux density on the surface = 2600 Gauss

Dead weight: 0.19 g Holding force on a steel plate: 4.64 Newton Weight, which the magnet can lift: 0.47 kg

Small displacement needs to be measured of around +-.3mm Sensor can be up to 3cm away (min 1mm) from the stationary point of the displacement. But there is a second margent not moving 1.5mm away from the first. Frequency of change between the max and min could be as high as 400Hz and displacement could be as low as +-0.05mm at times.

The sensor has to be of little effect on the system as possible although a small effect can be compensated for. System should be non contact to the moving base.

One idea that was suggested was a hall effect sensor. However such a large gauss with a small displacement and the other magnet nearby with such a large offset makes me think this idea is possibly not going to work although I could be wrong. The surface for sensing is the size of the magnet if using hall effect or inductance or a maximum of around 8.5mm x 15mm

Another two options suggested where a capacitive sensor as the moving part is grounded and conducting however there is also ground all around so good shielding would have to be implemented or some form of optical sensor probably based on intensity with fiber wires for the sending and receiving light using some form of modulation and filtering to keep costs down.

I am open to any other suggestions or where someone might know a good sensor suitable, circuit diagram or informational website. I know this system is a lot to ask.

If the displacement oscillates at a sinusoidal waveform the output should reflect this and be analog if possible. if there is a phase shift that shift would have to be known and frequency would need to remain the same but the amplitude does not have to accurately represent the total displacement.

Thanks for any ideas or suggestions

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    \$\begingroup\$ Smells a bit like homework. \$\endgroup\$ Dec 1, 2010 at 11:03
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    \$\begingroup\$ guess it does look kinda like homework. wish it was then i could at least ask a tutor or lecturer. Its for a project im working on. Thanks for the tag for electromagnetism didn't notice that one and it is one of the uses i thought of. \$\endgroup\$
    – razer_6
    Dec 1, 2010 at 11:39
  • \$\begingroup\$ What are you actually trying to do? Make a geophone/seismometer? \$\endgroup\$
    – Nick T
    Dec 1, 2010 at 18:01
  • \$\begingroup\$ not quite. similar to some ways to a Seismometer. I am trying to determine the waveform of the deflection of a vibrating beam via non contact methods. I have used a vibrometer for similar stuff in the past but they are very expensive and due to setup are not suitable outside of lab conditions and i do not need as precise a setup as that. \$\endgroup\$
    – razer_6
    Dec 1, 2010 at 18:08

1 Answer 1


You mentioned:

some form of optical sensor probably based on intensity with fiber wires for the sending and receiving light using some form of modulation and filtering

This is a good idea. You can do very precise measurements with lasers, measuring phase differences to get distance. This is sometimes done with the laser wavelength itself to get micron resolution; for larger resolution and range, you modulate the beam to a specific wavelength.
I'm not sure I understand the application entirely, but it seems like you will be measuring two different points at the same time, each with a different range. For a certain range, I would choose a wavelength at least 4x that -- this sets a maximum modulation frequency. For example, c/(4x6cm)=1.25GHz for the end of the beam. (You can use a much lower frequency, too). Resolution has a theoretical minimum set by the wavelength as well, but will be practically set by your detection setup. The idea is to compare the reflected phase with a reference.

Here's how Brad did it to pick up minute sound vibrations on surfaces far away -- a laser spy device: http://www.lucidscience.com/pro-laser%20spy%20device-1.aspx

  • \$\begingroup\$ Thank you Tyblu, Looks like it could be interesting. i will research that further. Sorry if i gave the impression of measuring two points at the same time. It should only measure one point. \$\endgroup\$
    – razer_6
    Dec 2, 2010 at 8:58

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