Sensor ensures proper clamp operation

Plymouth, MN--Before an automated system welds two parts together, it must "know" whether the parts are properly clamped. If they're not--and if the system cannot distinguish that fact--it can be a nightmare for manufacturers. Costly tooling can be damaged and assembly lines shut down, resulting in losses of tens of thousands of dollars.

To combat that, engineers from Turck Inc., have developed a patented sensor that attaches directly to a clamping arm's shaft. The new system reportedly is more compact and more accurate than previous clamp-sensing techniques. It also uses less hardware.

The key to the new sensing system is that Turck engineers placed the sensor on the clamp arm's shaft, instead of in the hydraulic or pneumatic cylinders that move the arm. "Ideally, you want the sensor as close to the action as possible," notes William Eaton, chief engineer for Turck and co-inventor of the system. "But it's not practical to put a sensor on the clamping arm. So the next best place is on the shaft that connects to the arm."

Up until now, sensors were typically placed at each end of the cylinder. When the cylinder's piston was at one end of its stroke, the sensor recognized it as clamped. At the other end, it was unclamped.

But by attaching the sensor to the clamping arm shaft, Turck engineers say they've realized several advantages. First, the sensor isn't subjected to high pressures within the cylinder. Nor is it exposed to hydraulic or lubricating oils. Furthermore, it's more accurate, they say.

To accomplish all that, the new system, known as the Turck PCS sensor, "watches" the angular displacement of the clamping arm, rather than the linear displacement of the piston. It includes a stationary sensor housing and a rotating element, which moves with the clamping arm's axle.

To measure angular displacement, the rotating portion of the sensor employs a pair of mechanical programming wheels, which contain small metal sensing targets. During operation, stationary sensors in the housing detect the presence of these targets. To determine when the clamp arm is retracted, one inductive proximity sensor detects the presence of a target on the so-called "lower programming wheel." To determine when the clamping arm is extended, another sensor detects the presence of a target on the upper programming wheel. The sensors attach to a single cable, which sends their signals back to a programmable logic controller or a computer. As a result, the system "knows" if the arm is retracted or extended. Hence, it also "knows" if the part to be welded is clamped or unclamped.

One of the beauties of the design is that the system's programming wheels and sensors can be used for a wide variety of clamp openings, ranging from 18 to 118 degrees. In the past, clamping sensors typically required a variety of probes and probe lengths. The new system also eliminates the use of special ports in the hydraulic or pneumatic cylinders for placement of the sensors. As a result, those ports can now be used for ducting of air or oil. The new design also reduces hardware: Because it needs only a single cable, the PCS sensor eliminates the need for an extra cable and extra connector.

During operation, the new sensing system also offers another advantage: In the unlikely event that a clamping arm breaks, the system will "know" it. In contrast, sensors placed on or near the piston have no way of sensing a broken clamp, because they monitor movement of the piston, not the clamp. Because the new technique monitors movement of the clamp itself, however, it immediately senses if the clamp is incapacitated. For many users, that may be the key advantage of the technology, because it enables them to stop the transfer line before parts are damaged.

The sensor design does require a specially designed clamp, however. Turck has teamed with De-Sta-Co Industries (Madison Heights, MI) to incorporate the sensing mechanism on clamps made for the auto industry.

For the user, the system's main advantage is that it improves the accuracy of detection. "This system is virtually foolproof," Eaton says. "You don't have to tighten it to a certain torque and you don't have to worry about pressures. You install it with three screws, connect it to your cable, and you're done."

Other Applications

- Automotive

- Material handling

- Aircraft