Vibration Sensor - Balancing Machine Im' working on a balancing machine. this machine has two vibration sensors. (pizzo) my question is about that why this machine use two Sensors? this machine has an rotary encoder to detect disk angle. 

i trying to read these signal by ADC card and using FFT bandpass filtering to calculate unbalanced mass on disk. for this algorithm i just use one of Vibration sensors. should i use second sensor?
Of course, the machine could be provided with three, four or more sensors, if the goal of the control unit is perform a balancing of the rotation. Actually three sensors should be the proper selection if the rotation is unconstrained in a biaxial plane, and two sensors, if constrained in a uniaxial direction.