How to detect a (very) small change in the position of a needle?
I would like to mimic the motion-sensing filiform hairs in spiders and insects.


I think you have unwittingly stated the reason your project may not be practical. I suspect that you will find that the insects and the piezos respond to changes in position and don't report position. In other words, a signal is given on and proportional to the change intensity or velocity but in the steady-state no signal is given. Run a finger along your arm with a light touch: you will sense the motion easily but not the steady touch. (This avoids nervous system overload otherwise every part of your body would be reporting constantly.)

In the same way a piezo crystal will give an output during change but if we draw the charge off by measuring it the voltage will drop to zero. To track position we would need to integrate the signal over time. This would not be an easy task with small signals and high impedances.



As a simple workable alternative I offer the following.

Standard industrial technique is mass flow measurement by hot wire. A quick web search gives the following from TiePie Automotive:


  he Mass Air Flow (MAF) sensor measures the amount of passing air going through the air intake system. The sensor measures the amount of passing air with a resistive platinum wire that is placed in the air stream of the air intake system. The resistive wire has a positive temperature coefficient: the higher the temperature the higher the resistance of the wire. Electronics in the air flow meter sends an amount of current through the wire to warm it up. The wire temperature is kept constant. When more air passes the wire, more current is needed to keep the wire at a constant temperature. The current that flows though the wire is a measure for the amount of passing air. The current is measured by the electronics in the sensor and converted to a voltage at the output of the sensor. The engine control unit (ECU) calculates the amount of air going into the combustion chambers from this voltage. 




Source: DieselNet.


  The hot wire air flow meter is the oldest type of anemometer, introduced into commercial automotive technology around 1985. The hot wire air flow meter (HLM) by Bosch proved highly popular. While no longer used in automotive applications, hot wire sensors are still installed in aeroplanes with piston engine drives.


The hot wire sensor has a simple functional principle and can operate without any moving parts. An electrically-heated wire (hot wire) is placed in the intake pipe after the air filter, transverse to the flow direction, Figure 1. A temperature sensor is installed upstream of the hot wire. The hot wire is then heated to a temperature higher than the air temperature determined by the temperature sensor. As the hot wire is cooled by the incoming air, the heating current must be increased to maintain a constant wire temperature. In this method, the heating current represents a direct measurement for the incoming air mass, independent of the air density. Thus, the reduced air density at high altitudes is elegantly compensated for.