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