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The cutting force can be estimated indirectly by monitoring the change of the motor current since it is related to the cutting force of cutting process. Motor current is proportional to the output torque of the motor that produces the amount of the cutting forces, which provide the needed mechanical force to remove material from the workpiece. Thus, the motor current is very useful for the tool wear, breakage/failure detection, and quality prediction of workpiece.

Current measurement sensors can be used to monitor the cutting process in a manner similar to a tool dynamometer. The Hall effect current sensor, or the so‐called current transducer (CT), converts motor current into an output voltage without any physical contact through detecting changes in the electromagnetic field, which is caused by the motion electric charges. As illustrated in Figure 2.5, a CT detects the electromagnetic field in the air surrounding when the electric current passes through a power cable.

Figure 2.5 Installation of a CT.

Compared to traditional sensing techniques based on Ohm’s Law, which generates a voltage that is proportional to current by directly connecting a resistor in series with the circuits, the Hall‐effect CT sensor does not disturb the machining since it has no direct connection to the circuit that carries the current.

The installation location can be attached directly on the motor cables without fixed costs. This isolated or noninvasive advantage makes the Hall current sensor become the most popular measurement method to indirectly sense the motor power or spindle currents with the sampling rate ranging from 100 Hz to 10 kHz. In addition, a Hall current sensor is available at a very low cost and is extremely durable. However, it is important to note that the unrelated magnetic objects have to be kept away from the operating environment since the Hall current sensor is vulnerable to magnetic fields.