Sensor activated downhole cutting tool

A downhole tool, comprising:

• a body having a bore extending at least partially therethrough;
• a sensor assembly in the bore, the sensor assembly including:
• a housing made of a non-magnetic material, the housing defining an interior chamber;
• a shaft coupled to the housing;
• at least one bearing around the shaft, the at least one bearing including a journal bearing having an inner sleeve and an outer sleeve, a radial clearance between an outer diameter of the inner sleeve and an inner diameter of the outer sleeve being between 1 μm and 1 mm;
• an impeller around the shaft and the at least one bearing, the impeller including a magnetized portion;
• and
• a sensor within the interior chamber, proximate to the magnetized portion, the impeller, or both, the sensor being configured to detect the magnetized portion;
• an electronics assembly in the bore and configured to receive and decode at least one signal from the sensor assembly indicative of a rate of rotation of the impeller, an axial distance between the impeller and the sensor, or both;
• and
• an actuation assembly in the bore and configured to actuate a mechanical device between at least two states in response to the at least one signal.

A downhole cutting tool, comprising:

• a body having a bore therein;
• one or more expandable cutter blocks having a plurality of cutting elements thereon;
• a sensor assembly in the bore, the sensor assembly including:
• a housing having an interior chamber;
• a shaft coupled to and extending axially from the housing;
• a polycrystalline diamond bearing around at least a portion of the shaft, the polycrystalline diamond bearing including an inner sleeve and an outer sleeve, a radial clearance between an outer diameter of the inner sleeve and an inner diameter of the outer sleeve being between 1 μm and 1 mm;
• an impeller around the shaft and the at least one bearing, the impeller being outside the housing;
• and
• a sensor within the interior chamber the sensor being configured to detect movement of the impeller;
• an electronics assembly in the bore and configured to receive and decode at least one signal from the sensor assembly indicative of a rate of rotation of the impeller, an axial distance between the impeller and the sensor, or both;
• and
• an actuation assembly coupled to the housing and configured to move the one or more expandable cutter blocks between inactive and active states in response to the at least one signal.