A retracting needle assembly for use with a syringe barrel having a distally-facing annular sealing surface includes an outer hub having a conduit therethrough, a proximally facing annular sealing surface and means for connecting the outer hub to the syringe barrel. The first measuring apparatus includes a first sensing device having a sensing field for obtaining positional data of a first testing target, a calibration target rigidly linked to the first sensing device, wherein the positional relationship between the first sensing device and the calibration target is known. The motor thereby achieves reduction in thickness, current, and cost, and inhibits non-repeatable runout. A bottom plate is welded to an outer bottom surface of the can, and a lead plate having a first end welded to the outer bottom surface of the can and a second end connected to a safety device so that the can is electrically connected to the safety device. Further, by calculating the gravity center acceleration not affected by the rotation, it is possible to detect a fall of the device with high precision even if the device rotates as it falls.