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To measure process accuracy or production quality of a process tool, direct and indirect techniques may be applied. For direct techniques, the process accuracy or production quality can be measured in the machine by various sensors such as touch sensor, charge‐coupled device (CCD), laser detector, and ultrasonic sensors. However, direct techniques are limited in practice due to extreme environment of machine workspace (such as affected by cutting fluid and chips). Furthermore, valuable production cycle time is reduced along with the measurement of device accuracy or workpiece quality. Also, these sensors are usually very expensive and difficult to apply to the in‐line production due to the increased cycle time.

Relatively, indirect techniques for measuring process accuracy or production quality are less‐accurate. However, using the sensors to sense indirect factors (such as force, vibration, temperature, and power consumption) is more economical and feasible for achieving the purpose of on‐line and real‐time diagnosis and prognosis.