A robotic assembly is configured to support, insert, retract, and actuate a surgical instrument mounted to the robotic assembly. The robotic assembly includes an instrument holder base member, a motor housing moveably mounted to the instrument holder base member, a carriage drive mechanism operable to translate the motor housing along the instrument holder base member, a plurality of drive motors, a plurality of gear boxes, and a plurality of output drive couplings driven by the gear boxes. The robotic assembly includes a sensor assembly that includes an orientation sensor, a sensor target, and a sensor shaft drivingly coupling the sensor target to a corresponding one of the output drive couplings.