The USi-safety ultrasonic sensor meets safety standards in accordance with Cat. 3 PLd, even in harsh environments. A detached sensor system provides installation flexibility—the miniature sensing units fit in tight spaces. It offers reliable safety operation regardless of the target material, surface structure and surface color. Optimal safety protection is assured by a “wide and shallow” elliptical 3-D detection field. It has easy teach-in of reference target for high tamper protection.