Assembly Innovations: Ultrasonic Sensing System Ensures Correct Fastening Sequence
Whenever an operator must install several fasteners in a part at an assembly station, the issue arises: How do you ensure that the fasteners are tightened in the correct sequence?
In some cases, the installation sequence doesn't matter. The fasteners can be run down in any order. In other cases, such as joints that include a compressible gasket, the sequence is critical. Installing the fasteners in the wrong sequence could deform the gasket and create a leaky joint.
During the past few years, tool suppliers have devised a variety of products to address the problem. For example, some suppliers offer support arms equipped with rotary encoders and other position sensors. The sensors tell a computer the location of the tool, and the computer sounds an alarm if the operator strays from the desired sequence. Another option is to pair a computer-controlled DC electric nutrunner with a flat-panel display. Text, drawings or photos can then show the operator the correct sequence. Alternatively, the task can be taken out of the operator's hands altogether. DC electrics can be fixtured together, mounted to a linear actuator, and programmed to drive the fasteners in the correct sequence.
Now, sensor manufacturer Pepperl+Fuchs Inc. (Twinsburg, OH) has introduced its own take on the problem-the UOS-100 ultrasonic positioning system. The system can determine the position of a fastening tool, in three dimensions, to within less than 1 inch. When used in conjunction with a computer or PLC, the system ensures that a tool can only be activated when a fastener is being installed in the right location at the right time.
The system consists of three main components: an ultrasonic pulse transmitter attached to the tool; three or more receivers located throughout the work area; and a control unit. Each controller can handle four transmitters and 16 receivers, which have a range of 50 feet. This enables one controller to monitor four tools over a relatively large area.
The system locates the tool by triangulation. The controller sends an activation signal to a transmitter and starts a timer. The transmitter emits a high-frequency sound pulse that travels in nearly all directions. Receivers mounted in the vicinity receive the pulse and send a signal back to the controller. Using the timer, the controller determines how long it takes for the sound pulse to travel from the transmitter to all the receivers. Next, the distances between the transmitter and the receivers are calculated, and the location of the tool in space is determined. The entire process takes approximately 100 milliseconds.
The system can be used to ensure that the correct number of fasteners are installed at an assembly station and that the fasteners are tightened in the right sequence. It can also be set up to limit the number of assembly stations at which a tool can be used. The system also enables assemblers to link torque and angle data to fasteners at specific locations.
Volkswagen (Wolfsburg, Germany) has already implemented the technology at several assembly plants, says Helmut G. Hornis, Ph.D., manager of the Intelligent Systems Group at Pepperl+Fuchs. The system is being used at assembly stations for seats, seat belts, air bags and motor mounts.
For more information on ultrasonic positioning systems, visit www.am.pepperl-fuchs.com, or eInquiry 21.