Without complex image recognition software will have trouble tidying away objects successfully. They will need to use multiple sensors, working together, to identify objects. Sensors could detect different colors, heights and widths. Dimensions could be determined by moving ultrasonic sensors. However, if multiple objects were close together it may be impossible to determine how big or small an object is. Also, a moving object (like a person or the cat) may confuse the robot.

Some objects with curved surfaces (like a drinks can) may be a problem for ultrasonic sensors like the one that comes with the Lego NXT kit. This is because the ultrasonic sound waves may bounce off the sides of the can and not bounce back to the ultrasonic sensor.