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How to build a shaker measurement setup to determine accelerometer sensitivity?

We will explain the rudimentary idea of how to determine the sensitivity of a piezoelectric accelerometer.
Such a setup requires 3 parts:

  • Shaker
  • Reference accelerometer
  • Accelerometer circuit

The shaker circuit consists of a shaker, function generator to create the desired waveform and in moste cases an amplifier since the function generator will not have the power output required by the shaker.

Assuming the reference accelerometer is also a piezoelectric accelerometer, you would nee to amplify and convert the charge signal to a voltage signal so the acceleration can be determined. This can be done by using a charge amplifier. A charge amplifier in its most basic form consists of a range capacitor, time constant resistor, operational amplifier and its power supplier. The range capacitor and the time constant resistor represents the cut-off frequency of the system. The gains is the ratio between internal crystal capacotance and the range capacitance.

The accelerometer for which the sensitivity needs to be determined hsould the be placed on top of the reference accelerometer. Similar charge amplifier should be used. The voltage signal can be read in an oscilloscope. It is generally advised to measure the acceleration at different frequencies. If nothing is specified 1 kHz can be used.

Since accelerometers work outside of its resonance it is obvious that one should stay away from their resonances. It is generally advised to avoid the shaker's natural frequencies too.

How to solder wires to silver electrodes?

Soldering electrical wires to screen-printed silver electrodes makes excellent and time-stable connections. Occasionally there can however be problems with wetting the solder on the silver surface and soldering can therefore be difficult. This phenomenon is mainly caused by a reaction between sulphuric molecules in the atmosphere with the silver surface, and the following formation of a silver sulphide layer on the surface of the part. The formation and thickness of this layer is influenced by several factors such as time, pH, humidity, etc. The solderability and electrode adherence is checked by our final inspection unit for every order (Military STD 1376A), but since this is performed under standard conditions, the solderability might change when parts are used by a customer.

In order to avoid such problems completely at any time, it can therefore be considered as good practice to gently clean the external electrodes on the part before soldering. A glass brush or steel wool is very useful for this operation. When soldering is performed, it is important that the temperature does not exceed the Curie point of the material, since this will immediately depole the piezoelectric phase. For normal PZT types we recommend to use soldering temperatures between 250 and 300° C. Silver is furthermore soluble in the solder, and if the solder time is too long, the electrode will completely dissolve in the solder. In order to increase the possible solder time we therefore recommend using solder with a silver content of 2-3%. Type SN62 from Multicore is for example a normal choice. Even if the possible solder time is increased with this type of solder, we recommend that the solder time does not exceed 2-3 seconds.

It should also be noted that the piezoceramic parts do not only have high piezoelectric coefficients, but also significant pyroelectric coefficients. This means that the increase in temperature introduced by the soldering process will generate an electrical charge. This charge can be released as a spark, which, even if completely harmless to humans, can be very unpleasant for the operator. It could therefore be recommended to solder parts in short-circuit conditions. Finally the below figures illustrate good practice in the orientation of the wires on the surface of the ceramic. Using such configurations will secure the maximum pulling strength of the wire.

 

How to solder wires to silver electrodes