An improved pencil sharpener assembly is provided for construction applications. The WWAN and the WLAN connect to a control center. The invention’s protective SiON or PE-OX DARC layer is formed over the dielectric layer whereby the protective SiON or PE-OX DARC layer fills in the microscratches. Bias could be applied to the third magnetic sensing unit to cancel a component of the magnetic field sensed by the third magnetic sensing unit so that the third magnetic sensor unit only provides sensing in the third direction. The phase comparator receives the first determining signal and the second determining signal for outputting the tracking error signal according to a phase difference between the first and the second determining signals.