**3.1 Accelerometer sensor**

This type of sensor is used for general purpose applications for vibration and shock measurements. They are available as digital or analog devices and designed using different principles such as piezoelectric, piezoresistive and capacitive. These components are used to convert the mechanical motion caused in the device into an electrical signal. Accelerometers are the most used vibration sensors for predictive maintenance of turbines, pumps, motors, and gearboxes. The piezo vibration sensors are considered as the gold standard in this category. An important category for acceleration measurements are the MEMS sensors. MEMS accelerometers and microphones are highly suited to battery-powered predictive maintenance systems due to their small size, low power consumption, and high-performance capabilities [16].

### **3.2 Strain gage sensor**

Strain gage is a sensor whose resistance changes with an applied force. From resistance change of the *strain gauges* a voltage change can be obtained with an electrical conditioning circuit such a voltage divider or Wheatstone bridge. This voltage is *measured* and is related to the *vibration* components. The amplitude and frequency of the *vibration* are changing during a machine's operation. These sensors have the advantage over many other types that they can be used for curved surfaces.
