Metalforming Electronics



Is Your Ram on the Level--Part 2?

By: George Keremedjiev

Tuesday, March 1, 2011
Last month we looked at ram parallelism and some basic checks that a metalformer can conduct to ensure its presses meet the specifications from the press manufacturer. Now let’s take a look at how stampers can use sensors to actually track ram travel over a specific distance. With this information, the stamper can check if the ram is moving in a predictable linear path, or if it is wobbling during travel.

To track ram travel, we can set up a fixture containing four analog LVDT sensors, with each sensor monitoring one corner of the ram’s travel. The ram could be unloaded or it could have an upper die section attached.

LVDT sensors come in many sizes, and resemble an air cylinder in appearance—a cylindrical body and a rod that travels in and out of the body. The LVDT reports this rod travel as a variable voltage that can be monitored by a personal computer (PC) with the appropriate analog input card. To monitor ram travel, the stamper must specify the LVDTs to cover the ram travel distance of interest. For example, consider monitoring ram travel during its last 1⁄2 in. of motion down and up. The LVDTs would be internally spring loaded and would have the ability to monitor at least a 1⁄2 in. of travel.

The fixture housing the sensors must be carefully mounted using parallels or some other supporting mechanism to ensure that the sensors monitor the ram where required. To prevent the LVDTs from being destroyed by an inadvertent over-travel by the ram, due to poor setup, we embed each LVDT within a spring-loaded block that travels into the fixture should ram travel exceed the LVDTs’ range, protecting the sensors. LVDTs are not very forgiving of lateral forces applied to their moving rods, so stampers must take care to avoid any sides pressure.

Back to our example—As the ram travels, distance measurements are reported to the PC by the four LVDTs, with a resolution well below 0.0001 in.

As an alternative to LVDTs, stampers can install spring-loaded rectangular probes, each with one of their sides machined to a taper. As the probes move down and up with the ram, the four tapered sides are monitored by four analog inductive proximity sensors whose output would be sent to a PC. The stamper must be sure to select and test the proximity sensors to ensure they have sufficient linearity to accurately report the travel of the probes over the full range of interest.

Yet one more method used to monitor full ram travel, via its four corners, is by using magnetostrictive sensors. These have a cylindrical body from which a hollow sealed tube extends, in a variety of lengths spanning a few inches to several feet. A round doughnut-shaped magnet travels (non-contact) along the length of the tube, its position reported by the sensor to the PC. Magnetostrictive sensors are absolute—they report magnet position as a true position, not a relative value. This allows extremely accurate monitoring of ram travel during its full down and up motion.

In each of these cases, the PC would display the outputs of the four analog sensors and could be programmed to display the four travel paths as graphs. The PC also could be programmed to look for deviations from a linear travel path, report any anomalies and allow the stamper to note wobbling conditions.

Note: Employing the above-described procedures requires careful design, mounting and use of fixturing, while adhering to applicable safety practices and regulations. There are no short cuts when dealing with a power press in motion. MF


Related Enterprise Zones: Sensing/Electronics

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