An apparatus and method for calibrating or teaching a robot, the apparatus includes a reflective photoelectric sensor arranged on a gripper of the robot and a controller. The controller is configured to: cause the reflective photoelectric sensor to scan over a target object; monitor changes in an output signal from the reflective photoelectric sensor; for each detected change exceeding a threshold, determine a coordinate of a gripping component on the gripper in a robot coordinate system, to obtain a set of coordinates; determine a position of the target object in the robot coordinate system based on the set of coordinates and a predefined offset value (PO) between the reflective photoelectric sensor and the gripping component; and store the position of target object for future use in assembling objects.