Table 1.

Suitability matrix of NII techniques for different applications.

Technique	Key Characteristic	Limitations	Application
Contact-Based Techniques
Wear debris [118,119,120,121]	Very accurate at establishing severity Low cost	Cannot locate faults Interrupts operation	Machine lubricant oil analysis
Vibration sensing [109,110,116,117]	Good response Withstand high temperatures High accuracy	Contact-based Sensitive to machine noise	Machine body and equipment measurement
Flux radiation or voltage and current [92,93,94,97,98]	Low cost Fast action Simple installation	Suitable for motors but not fuel-powered engines Impacted by a change in the supply network Weakness in tracking load change Cannot quantify severity of damage	Motor body and wiring
Non-Contact Based Techniques
Ultrasonic sensing [57,58]	Insensitive to weather conditions Senses all materials Larger size provides better sensing	Sensitive to temperature changes Struggles to read small object reflection	Anti-collision Doors
Imaging camera [63,64]	non-contact No harmful radiation Operates in real-time	Heavy and large size Impacted by weather and dust High usage of data	Image monitoring Machine vibration Car speed detection
Thermographic sensing [83,84,85,86]	No- harmful radiation Life-time operation Specify fault area	Temperature interference from others surfaces	Human body detection Machine and equipment heat
Radar sensing [46,48,49,50,51,52]	Insensitive to weather conditions Multiple objects at a time Easy installation Fast data acquisition Longer measuring range Detect different types of materials	High cost No fault classification Signal spreads Hard to classify close objects	Detect objects and motions Object speed and size Car’s sensors Autonomous car Door
Acoustic emission [67,72,73]	Non-contact High sensitivity Low cost Real-time monitoring	Noise impact No fault classification	Music ditemize Machine noise sensing
Laser sensing [89,90,91]	Non-contact Multiple object detection Long measuring range	No fault classification Cost Eye risk Affected by the weather	Machine vibration sensing