To speed automobile assembly lines, Radix Controls (Oldcastle, ON, Canada; www.radixcontrols.com) has automated the torquing of wheel lug nuts during production. The company uses a vision system to determine the position of the wheel, including its fore and aft and side-to-side positions, and three rotational axes. With this information, a robot moves the nut runner into position to tighten the nuts.
This is a difficult manual job because of the size and weight of the nut runner and the need to tighten the nuts on two wheels in approximately 40 s. A vehicle is only roughly positioned by a conveyor, and the wheels themselves are free to rotate, tilt, and turn.
The application relies upon two Cognex In-Sight 5403 vision systems to locate each wheel. After the system identifies the type of wheel, the circle finder in Cognex’s PatMax software is used to determine the center of the axle. Edge tools inspect the center of the rim to determine the angle of rotation of the wheel. This information is sent to the controller of a Fanuc robot, which matches the angles to which the wheel is tilted and turned and rotates the nut runner to match the wheel’s angle of rotation. The nut runner is then guided squarely onto the lug nuts and tightens them to the proper torque.
The robot must also be calibrated to the vision system. The work cell is fully calibrated in less than one minute with the custom features on a specialized calibration target, and completes the automatic dynamic calibration sequence in less than 2 s per cycle.