Piezo Accelerometer Tutorial
The charge output of a piezoelectric sensor is a very particular signal and cannot be connected to any desired device without impact on the signal itself.
That is why we need an adapted signal conditioner, a device that converts the charge signal so it can be shared and processed further.
Why would we need a charge amplifier? Could we not just as easily use a standard voltage amplifier?
When we use a cable to connect the piezo to the voltage amplifier we add an electrical capacity Ccable. Together with the capacity Cpiezo of the piezo element the total capacity becomes then Ctotal = Cpiezo +Ccable. The increase in capacity means that the voltage of the piezo element drops significantly. In addition when we use a cable with a different length the voltage would change again so the sensitivity of the set-up would not be known.
This problem can be avoided by using a charge amplifier. Correctly speaking it does not amplify the charge but it is rather a charge-to-voltage converter meaning it puts out a voltage which corresponds to the applied electrical charge at the input. The output of a charge amplifier is a standard electric signal which can be connected to other devices also over longer distances. On the input side we should be more careful because the input circuitry is normally sensitive to high voltage. That is why we always short the input when we are fiddling around the input of a charge amplifier.
Charge amplifiers work over a large frequency range including lower frequencies. However if we are looking for extremely low frequencies a good total insulation resistance of the connecting cable and high internal resistance of the accelerometer are required. Otherwise the output signal may start to drift into saturation. Although we can measure at very low frequencies a charge amplifier is not suitable for measuring static charges. Normally there is a possibility to choose a time constant which dictates the behavior at low frequencies.
Due to the time constant the output will be pulled towards zero when the frequency goes too low. Most laboratory charge amps have also a “quasi static” setting which allows theoretically static signals at least for a short time. However this needs extremely high insulation resistances and also a critical view at the result.
In practice, a charge amplifier contains also additional circuit stages, such as high and low pass filters, integrators and level control circuits, as well as additional output stages to provide some standardized voltage or current output signal.
Integrated Electronics (IEPE)
In view of signal noise and ease of signal transmission a good place to put an amplifier is in the accelerometer itself. In fact it is quite common to place a small electronic inside the sensor housing when specifications like temperature and other environmental conditions allow. This technique is called IEPE and stands for Integrated Electronics Piezo-Electric but there are also registered trade names like ISOTRON (Endevco), ICP (PCB), CCLD and DeltaTron (both B&K) or Piezotron (Kistler). The IEPE sensor electronics converts the high impedance signal of the piezo element into a low impedance voltage signal. Low impedance means a more robust output which allows using longer cables and less noise by electromagnetic interference.
With the conditioner mounted inside the sensor there is of course no cable from the piezo element and no problem related to the capacity. The complete IEPE circuit comprises an external constant current source as a power supply, which is located at the far end of a coaxial connection cable. Through the center conductor both the supply current and voltage output are transmitted. The common return is through the cable shield. Typical supply currents are in the range of 2 to 20 mA.
The cable doesn’t need to be especially low noise like for a charge transmission. A standard coax will do it. However if we want to use very long cables (let’s say more than 10 meters) the response for higher frequencies (let’s say over 10 kHz) may be affected when the supply current is insufficient to handle the cable capacity.
The suppliers normally provide nomograms to determine the limit frequency in view of cable capacity and supply current. We can retain: The higher the supply current the longer the possible cable length.
The IEPE circuit is extremely simple. The sensor built-in part of the most common design consists in just one simple MOSFET transistor. At the same time a robust signal transmission with only few restrictions is provided. Due to the simplicity these circuits are very reliable also in rough conditions. There are IEPE sensors with surprisingly extreme temperature capabilities from cryogenic (5°K) up to 175°C or even 200 °C are quoted.