The sinusoidal output of piezoelectric accelerometers includes positive and negative components resulting from bidirectional motion imparted into the sensor axis. For clear understanding, conventions must be set for the motion and for the electrical charge. Axial motion imparted from the base of the sensor, directed into the sensor is considered to be in the positive direction. Motion directed away from the sensor base is considered the negative direction. This motion is transferred to the sensor’s seismic mass which loads the piezoelectric sensing elements providing a high impedance electrical signal known as charge output.
Charge output accelerometers are typically wired with positive motion resulting in a negative output as they are typically converted to low impedance signals via an inverting op-amp. This convention makes charge sensors negative polarity unless specified otherwise.
ICP® accelerometers have a built in externally powered microelectronics to convert the high-impedance charge signal into a low-impedance voltage signal directly usable by most modern instrumentation without requiring external charge amplifiers. For this reason, ICP® sensors are typically positive polarity.
Note about charge amplifiers: different manufacturers have used both inverting and non-inverting op-amps within their amplifier electronics and it is important to understand which type of amplifier you are using with which type of sensor. PCB inline charge amplifiers are of a negative polarity, designed for use with standard charge sensors resulting in a positive signal to readout devices.
Charge sensors, in an inverting design, are labeled with a P-prefix to indicate the special positive polarity. Be aware that some custom ‘M’ models include the inverting functionality but do not have the P-prefix. Inverted ICP® sensors are extremely rare. Refer to the accelerometer datasheet to confirm if the specific model has P options available and confirm the P-prefix in the specific sensor’s model number.
Accelerometer output can be confirmed through use of an oscilloscope (possibly with use of a volt meter for sensors with relatively long time constants), hold the sensor in the palm of your hand, and tap gently on the accelerometer’s base (positive input). Similarly, polarity of ICP® sensors can be confirmed using a PCB battery operated signal conditioner where the needle will move to the right indicating positive polarity and move to the left indicating negative polarity.
All of this holds true for triaxial sensors that receive symbols etched into each sensor detailing which direction is positive for each axis (x, y, and z). There are specific applications where knowing the specific polarity has a greater impact – predominantly tests with multiple sensors or inputs at multiple locations such as modal analysis. Incorrect polarity, direction or inconsistent sign conventions will result in inaccurate results.